Increased Incidence and Clinical Picture of Childhood
Narcolepsy following the 2009 H1N1 Pandemic
Vaccination Campaign in Finland
Markku Partinen1,2,15*, Outi Saarenpa ¨a ¨-Heikkila ¨3, Ismo Ilveskoski4, Christer Hublin5, Miika Linna6,
Pa ¨ivi Olse ´n7, Pekka Nokelainen8, Reija Ale ´n9, Tiina Wallden10, Merimaaria Espo10, Harri Rusanen11,
Jan Olme12, Heli Sa ¨tila ¨13, Harri Arikka14, Pekka Kaipainen15, Ilkka Julkunen16, Turkka Kirjavainen17
1Helsinki Sleep Clinic, Finnish Narcolepsy Research Centre, Vitalmed Research Centre, Helsinki, Finland, 2Department of Clinical Neurosciences, University of Helsinki,
Helsinki, Finland, 3Unit of Child Neurology, Department of Paediatrics, Tampere University Hospital, Tampere, Finland, 4Department of Child Neurology, Children’s
Hospital, Helsinki University Central Hospital, Helsinki, Finland, 5Finnish Institute of Occupational Health, Helsinki, Finland, 6Department of Statistics and Registers,
National Institute for Health and Welfare (THL), Helsinki, Finland, 7Department of Child Neurology, Oulu University Hospital, Oulu, Finland, 8Department of Child
Neurology, Kuopio University Hospital, Kuopio, Finland, 9Department of Child Neurology, Jyva ¨skyla ¨ Central Hospital, Jyva ¨skyla ¨, Finland, 10Department of Child
Neurology, Central Hospital of Kymenlaakso, Kotka, Finland, 11Department of Neurology, Oulu University Hospital, Oulu, Finland, 12Department of Child Neurology,
Vaasa Central Hospital, Vaasa, Finland, 13Department of Child Neurology, Kanta-Ha ¨me Central Hospital, Ha ¨meenlinna, Finland, 14Department of Child Neurology, Turku
University Hospital, Turku, Finland, 15Rinnekoti Research Centre, Espoo, Finland, 16Department of Vaccination and Immune Protection, National Institute for Health and
Welfare (THL), Helsinki, Finland, 17Department of Paediatrics, Children’s Hospital, Helsinki University Central Hospital, Helsinki, Finland
Background: Narcolepsy is a rare neurological sleep disorder especially in children who are younger than 10 years. In the
beginning of 2010, an exceptionally large number of Finnish children suffered from an abrupt onset of excessive daytime
sleepiness (EDS) and cataplexy. Therefore, we carried out a systematic analysis of the incidence of narcolepsy in Finland
between the years 2002–2010.
Methods: All Finnish hospitals and sleep clinics were contacted to find out the incidence of narcolepsy in 2010. The national
hospital discharge register from 2002 to 2009 was used as a reference.
Findings: Altogether 335 cases (all ages) of narcolepsy were diagnosed in Finland during 2002–2009 giving an annual
incidence of 0.79 per 100 000 inhabitants (95% confidence interval 0.62–0.96). The average annual incidence among
subjects under 17 years of age was 0.31 (0.12–0.51) per 100 000 inhabitants. In 2010, 54 children under age 17 were
diagnosed with narcolepsy (5.3/100 000; 17-fold increase). Among adults $20 years of age the incidence rate in 2010 was
0.87/100 000, which equals that in 2002–2009. Thirty-four of the 54 children were HLA-typed, and they were all positive for
narcolepsy risk allele DQB1*0602/DRB1*15. 50/54 children had received Pandemrix vaccination 0 to 242 days (median 42)
before onset. All 50 had EDS with abnormal multiple sleep latency test (sleep latency ,8 min and $2 sleep onset REM
periods). The symptoms started abruptly. Forty-seven (94%) had cataplexy, which started at the same time or soon after the
onset of EDS. Psychiatric symptoms were common. Otherwise the clinical picture was similar to that described in childhood
Interpretation: A sudden increase in the incidence of abrupt childhood narcolepsy was observed in Finland in 2010. We
consider it likely that Pandemrix vaccination contributed, perhaps together with other environmental factors, to this
increase in genetically susceptible children.
Citation: Partinen M, Saarenpa ¨a ¨-Heikkila ¨ O, Ilveskoski I, Hublin C, Linna M, et al. (2012) Increased Incidence and Clinical Picture of Childhood Narcolepsy following
the 2009 H1N1 Pandemic Vaccination Campaign in Finland. PLoS ONE 7(3): e33723. doi:10.1371/journal.pone.0033723
Editor: Benjamin J. Cowling, University of Hong Kong, Hong Kong
Received November 9, 2011; Accepted February 15, 2012; Published March 28, 2012
Copyright: ? 2012 Partinen et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: The authors have no external support or funding to report. The collection of data from the registries and from different hospitals was funded by THL.
The study group had freedom for the study design, data collection, data analysis, data interpretation, and writing of the report. The authors had access to all the
data, take responsibility for the integrity of the data and the accuracy of the analysis, and had final responsibility for the reporting of the data.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: firstname.lastname@example.org
The reported prevalence of narcolepsy-cataplexy among adults
in Finland is 26 cases/100 000 inhabitants (95% confidence
interval CI of 0 to 60) . According to Silber et al. the incidence
is estimated to be 0.74 per 100 000 person-years for narcolepsy
with cataplexy and 1.37 per 100 000 person-years for narcolepsy
with or without cataplexy . The most common symptoms
of narcolepsy are unintended sleep episodes, excessive daytime
sleepiness (EDS) and cataplexy [3,4]. Most often narcolepsy starts
between 12–25 years of age with the main peak of onset at 14–16
years. An onset before age of 10 years has been rare [2,5].
PLoS ONE | www.plosone.org1March 2012 | Volume 7 | Issue 3 | e33723
Typically narcolepsy-cataplexy is characterized by the lack of
hypothalamic hypocretin (orexin) production [3,6] and strong
association with HLA DR15 (DR2) and DQB1*0602 [3,6,7,8].
Among Caucasians over 90% of patients with narcolepsy-
cataplexy are HLA DQB1*0602 positive . In a recent Danish
study including a meta-analysis of seven studies the CSF-
hypocretin-1 was low or undetectable in 69–100% (overall
approximately 80%; 218/274) of patients with narcolepsy-
cataplexy . Similarly approximately 20% of patients with
narcolepsy without cataplexy had low CSF hypocretin. More than
97% of patients with low CSF-hypocretin-1 are HLA DQB1*0602
Genetic associations [10,11,12,13,14,15,16], presence of anti-
tribbles 2 antibodies [17,18,19] and other recent observations
[20,21] suggest that autoimmune mechanisms are involved in the
etiopathogenesis of narcolepsy [22,23,24]. Seasonality of onset
 and reports of an association to preceding streptococcal
infections [26,27] have suggested a link with upper respiratory
At the end of December 2009, a 7-year old boy consulted
physicians because of a recent onset excessive daytime sleepiness.
The H1N1 epidemic was ongoing and he had been vaccinated
with Pandemrix. He had no history of recent upper respiratory
tract infection or influenza-like illness (ILI). In February 2010 he
was diagnosed with narcolepsy. A possible causal relationship with
influenza and/or Pandemrix vaccination was suspected. Several
new cases of recent onset childhood narcolepsy were observed in
Pandemrix-vaccinated children, without previous history of ILI. In
Finland up to fourteen diagnoses were confirmed before August
2010, bringing forward a possibility of a novel environmental
trigger for increased incidence of narcolepsy in children .
Similar findings were reported from Sweden, and there was
a time-related association observed with influenza A (H1N1)
pandemic and H1N1 vaccination [28,29,30]. Due to the time-
related association with influenza vaccination, the Finnish health
authorities decided to cease Pandemrix vaccinations in August
2010 . A systematic study was started to recognize all
narcoleptic patients in Finland diagnosed in 2010, and to compare
incidence figures with the diagnoses made between 2002 and 2009
. A parallel study was focused to study the role of the AS03-
adjuvanted vaccine (Pandemrix) based on register data . Also
in Sweden, the government funded studies were initiated .
Parallel case reports were reported in France and Canada, where
Pandemrix or a similar AS03 adjuvanted vaccine Arepanrix was
used. A possible association with H1N1 vaccination and recent
onset narcolepsy was found in fourteen (9 children, 5 adults). In
eleven cases Arepanrix/Pandemrix had been used (6 children ,17
years and 5 adults). Two of the six children were from France, two
from Canada, one from Switzerland and one from UK . In the
present study, we report our findings on the change in incidence of
childhood narcolepsy. We also describe the clinical picture of post-
Pandemrix narcolepsy in children, and compare that to previous
descriptions of childhood narcolepsy.
This study has been approved by the Institutional Review Board
of the National Institute for Health and Welfare (THL). The
clinical part of the study is based on case history obtained by the
clinician authors. Most results are based on clinical examination of
patients that was part of normal diagnostic procedures. A written
informed consent was received from all patients for use of their
data, and for laboratory studies and examinations that were not
part of the diagnostic procedure or that were done after the
diagnosis of narcolepsy had been confirmed. Parents (legal carers)
signed the written informed consent on behalf of the minors/
children involved in the present study.
Collection of information of diagnosed cases of
In Finland the patients with specific diagnoses can be identified
from the national hospital discharge registry (HILMO) kept by the
National Institute for Health and Welfare (THL). This register
includes comprehensively discharge abstracts from public and
private patient care in hospitals and institutions in Finland. The
unique personal identification code made it possible to obtain
incident first-ever patient cohorts in 2002–2009 with narcolepsy
with or without cataplexy (ICD-10 code G47.4) as the main
diagnosis. Patients with a previously diagnosed of narcolepsy were
retrospectively analysed until January 1st, 1997 and incident
cohorts from 2002 to 2009 were included. Since there is
approximately 18-month lag in the complete HILMO registry
data, G47.4 diagnoses set during 2010 were collected by
contacting the same sources that are used to collect data for the
national registry. The included databases are based on discharge
data of all hospitals and other health care organizations
responsible for specific diagnosis of narcolepsy.
To verify the onset of symptoms and to gather information of
the clinical picture of the diagnosed patients in 2009–2010 all
available patient information was collected nation widely from
hospitals and sleep centres. In addition, child neurologists in
different hospitals were contacted to ensure that all patients had
been recognized. The status and date of Pandemrix influenza
vaccination was verified from the vaccination certificates that were
filled by health care professionals. According to national guidelines
all vaccinated individuals had received only one dose of
Pandemrix vaccine. The presence of symptoms and their onset
were obtained from medical records. They were based on face-to-
face interviews with a child neurologist and the parents and
children. The date of first contact with health personnel due to
EDS was verified from school nurses and health care centres.
Calculation of narcolepsy incidence rates for different
The incidence rates for narcolepsy were calculated by dividing
the number of yearly-diagnosed narcolepsy patients by the total
number of people in Finland in each age group during the same
year. The incidence rates in different age groups (,11, 11–16, 17–
19 and people aged 20 years or more) were calculated. We used
the age at the time of diagnosis to allow comparison with the data
obtained from the hospital discharge registry (which includes age
at diagnosis). The demographics of Finnish population were
obtained from the Statistics Finland.
Diagnosis of narcolepsy
The diagnosis of narcolepsy was based on the criteria of the
International Classification of Sleep Disorders (version 2, ICSD-2)
. Multiple sleep latency test (MSLT) was done for all children.
Analysis of cerebrospinal fluid (CSF) hypocretin-1 was done at the
Rinnekoti Research Centre (Orexin A RIA kit, Phoenix
Pharmaceuticals, San Mateo, CA). HLA-typing was done for
some patients although it is not an official ICSD-2 diagnostic
criterion. Other causes of EDS (e.g. sleep apnoea, delayed sleep
phase syndrome or sleep deprivation) as well as other neurological
disorders (encephalitis, encephalopathy, other neurological disor-
ders) were excluded by polysomnography, actigraphy, thorough
Incidence and Symptoms of H1N1-Related Narcolepsy
PLoS ONE | www.plosone.org2March 2012 | Volume 7 | Issue 3 | e33723
neurological examination, magnetic resonance imaging (MRI),
EEG, CSF examinations, blood tests, and other examinations
when necessary. A child neurologist always determined the
diagnosis of narcolepsy. Before the patient was considered to
suffer narcolepsy the diagnosis was verified by a panel of five
neurologists/sleep specialists (CH, MP, OSH, PO, TK).
Statistical analyses were performed with STATA 10.1 (Stata
Corporation, TX). The incidence ratios were calculated for each
year and the average incidence ratios from the period of 2002–
2009 were compared to those observed in 2010 (1stJan 2010 until
31stDec 2010). The 95% confidence intervals (95% CI) are given
for the average incidence rates for the years 2002–2009. Age is
given as years and decimals. For statistical comparisons of the
continuous variables parametric or non-parametric methods were
used according to the normality of the distributions.
Incidence of narcolepsy in 2002–2009
Altogether 335 cases of narcolepsy were diagnosed in Finland
during 2002–2009 giving an annual incidence of 0.79 per 100 000
inhabitants (95%CI0.62–0.96).Among adults $20 years of age, 281
new cases of narcolepsy (range 25 to 52 per year) were diagnosed
between 2002–2009 giving anaverageannualincidence of 0.87 (95%
CI 0.71–1.03) per 100 000. Twenty-eight (3 to 5 per year) cases of
narcolepsy were diagnosed among individuals aged 17 to 19 with an
average annual incidence of 1.79 (95% CI 1.49–2.09) per 100 000
(Figure 1, Figure 2, Figure 3, Figure S1, Figure S2). Twenty-six
patients were younger than 17. In that age group the average annual
incidence in 2002–2009 was 0.31 (95% CI 0.12–0.51) per 100 000
children. Only one child aged less than 11 years (a 9-year-old) was
diagnosed with narcolepsy prior to 2010 (Figure 1, Figure S1).
Incidence of narcolepsy in 2010
In 2010 altogether 101 persons were newly diagnosed for
narcolepsy giving an incidence of 1.88/100 000. Of these patients
65 were younger than 20 years of age (incidence rate 5.33/
100 000/year). Among adults $20 years of age (n=36) the
incidence rate in 2010 was 0.87/100 000, which equals with the
averageincidencefigure in2002–2009 (0.87).Among17to19-year-
olds it was 5.46/100 000 (3-fold increase). In 2010 altogether 54
cases of childhood narcolepsy were diagnosed in children and
adolescents (,17 years; Appendix S1) giving an incidence rate of
5.30/100 000, which is 17 times higher than the average incidence
between 2002 and 2009.Inchildrenaged ,11,theincidencerate in
2010 was 3.39/100 000 (1.89 in children aged ,8 and 7.56 in
children aged 8 to10). Even if the highest incidence figures were
seen in peripubertal and pubertal children aged 11 to 16 years (8.76
diagnoses per 100 000), the highest increase from previous figures
was seen in children aged less than 11 years of age (increase from an
average of 0.02 to 3.39 per 100 000 giving a 177-fold increase in
incidence; see Figure 3, Figure S2).
Figure 1. Number of new diagnoses of narcolepsy among children and adolescents aged under 20 years of age by year of
Incidence and Symptoms of H1N1-Related Narcolepsy
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Based on patient records 50 of the 54 children had received the
pandemic H1N1 vaccine (Pandemrix, GSK) 0–242 days (median
38 days; 95% CI 40 to 67) before the onset of EDS. In 4 of the 50
vaccinated children (Appendix S1) influenza-like illness (ILI) was
reported during the national H1N1 epidemic peak during the
weeks 43–48 in 2009 . However, laboratory confirmation of
influenza infection of ILI cases had not been done. No other
microbiologically confirmed infections were found in any of the 54
Clinical picture of children (,17 years) with H1N1
Fifty children and adolescents, aged under 17 (age limit of
paediatric care in Finland), had an onset of EDS after Pandemrix
vaccination, and had a diagnosis of narcolepsy in 2010. All
children were of Caucasian origin. There were 28 (56%) girls and
22 boys (Table 1). There were no significant differences in the
clinical presentation of narcolepsy between boys and girls. The
mean (6SD) age at the time of vaccination was 10.863.0 years
Figure 2. Number of new diagnoses of narcolepsy among adults aged 20 years or more by year of diagnosis.
Figure 3. Annual incidence of narcolepsy by age group and year of diagnosis.
Incidence and Symptoms of H1N1-Related Narcolepsy
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(range 4.5 to 16.1 y). The mean age at onset was 11.063.1 years.
The mean age at onset was 10.262.8 years in boys and 11.663.1
years in girls (P=0.055). Eighteen per cent of the children were
younger than 8 years, 24% were 8–10 years old, 38% were 11 to
13 years old and 20% were 14–16 years old at onset of symptoms.
None of the 50 children had a prior history of EDS, cataplexy, or
other symptoms of narcolepsy. The initial symptoms of narcolepsy
included EDS, reappearance of regular daytime naps, and
unintended sleep episodes (Table 1). Forty-seven of the 50 children
(94%) have developed cataplexy, which started 6 to 359 days after
the vaccination (median 77 days, i.e. 11 weeks). All children had
abnormal multiple sleep latency test (MSLT) with a mean sleep
latency of 1.861.4 minutes (95% CI 1.4 to 2.2) and with at least 2
(median 4; mean 3.860.9) sleep onset REM periods (SOREMPs).
Clinically significant sleep-related breathing disturbances were not
found. The mean apnea-hypopnea index was 0.4 (SD 0.4, range
0–1.4). In 11 out of 13 children CSF hypocretin-1 levels were
undetectable (below 10 pg/ml) and it was pathologically low in the
remaining two children (32 and 69 pg/ml; ICSD-2 criterion for
narcolepsy ,110 pg/ml). All three children without cataplexy had
undetectable CSF hypocretin levels. MRI was done in 34 subjects.
There was one arachnoidal cyst without clinical significance while
in other patients MRI was normal. One child had type 1 diabetes
and one had von Willebrand disease. They were both DQB1*0602
positive. Nine children (16.7%) had atopy and/or asthma and four
children (7.4%) had had problems with attention and hyperactivity
before onset of narcolepsy. Twenty-four (48%) children showed
behavioural changes or psychiatric problems (conduct disorders/
challenging and aggressive behaviour, self mutilation), which
needed psychiatric treatment after the onset of narcolepsy.
Table 1. Clinical findings.
Dauvilliers et al.
Aran et al.
et al. 201141
Han et al.
Number of subjects5061
51 30 29
Period of diagnoses (yr)1
Proportion of females28/50 (56%)N/A 22/51 (43%)18/30 (60%)8/29 (28%)
Had Pandemrix or Arepanrix50/50 (100%)6/6 (100%)N/A N/AN/A
Age of onset (yr), mean (SD) 11.0 (3.0) Not given10.3 (3.57)14.0 (3.0)***
Age at onset (yr) of cataplexy, mean (SD) 11.4 (2.8) Not given Not givenNot given 9.2 (2.0)
Days from vaccination to onset, mean (SD);
median; 95% confidence interval (days)
53.8 (47.1); 38; 40 to 67 Not given N/AN/AN/A
Age (yr) at diagnosis, mean (SD)11.6 (3.1)11.4 (4.6)11.8 (3.57)15.6 (3.1)***
Time (yr) from onset to diagnosis0.1 to 0.9 , mean 0.7 (SEM
0.03); 45 to 345 days
,1 Mean 1.5 (SEM 0.3) Not given1–2
Cataplexy 47/50 (94%) 6/6 (100%)51/51 (100%)18/30 (60%)**
Time (wk) from vaccination to cataplexy,
mean (SD); median; range
13.7 (10.6); 11;
0 to 51
6.5 (4.5)*; 4.5;
3 to 15
Months from onset of EDS to onset of
cataplexy, median; range
0;8; 0 to 10
In 70%# #2 (N=47)
29 to 2In 82%#2Not givenCataplexy
onset in all?
Hypnagogic hallucinations26/49 (53.1%)Not given33/50 (66%) 15/30 (50%) 17/29 (59%)
Behavioral problems 24/50 (48%)Not given26/39 (66%) 10/30 (33.3%)27/29 (93%)***
Sleep paralysis9/49 (18.4%) Not given28/51 (55%)***
Disturbed nocturnal sleep44/50 (88%)Not given 47/51 (92%)Not givenNot given
Rapid weight gain in the beginning#
BMI in kgm22, mean (SD)19.6 (4.1); (N=42) Not given25.2 (1.2)**;
Sleep latency (min) in MSLT, mean (SD)1.8 (1.4)Not given2.5 (2.5)4.0 (3.1)***
SOREMPS3.8 (0.9)Not given Not given3.2 (1.4)*
Short SL and $ $2 SOREMPs in MSLT100% (N=50)Not given 92% (N=39)90% (N=30) 96.5% (N=29)
CSF-hypocretin-1Mean (SD) 8.6 (20.2); (N=13)
Median (range) 0 (0–69)
Not givenMean (SD) 4.5 (7.9);
,110 pg/ml ;
1This table contains information of only those 6 subjects who were aged#17 years at diagnosis. The other 10 of the 16 subjects reported by Dauvilliers et al.201034were
older. They were not included to enable comparisons.
#We defined weight gain as an increase of body mass index (BMI) .5%. Aran et al40defined it as .4 kg weight gain. NC: narcolepsy and cataplexy; NwC: narcolepsy
without cataplexy. N/A: Not applicable; SD: standard deviation; SEM: standard error of mean. Statistically significant differences between our study and other studies are
**for P,0.01 and,
*for P,0.05. The numbers (N=) in parenthesis refer to number of subjects with data.
Comparison of the present study and published studies from literature for children aged#17 years.
Incidence and Symptoms of H1N1-Related Narcolepsy
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Psychiatric hospitalisation was necessary in four cases, and one
patient needed temporarily benzodiazepine and antipsychotic
treatment. No signs of central nervous system infection were found
in CSF, MRI or EEG. During the spring 2010 three children
received intravenous gammaglobulin without any clinical im-
provement. All 32 of the 50 HLA-typed children were positive for
DQB1*0602/DRB1*15/DR15 – DQ6 genotype. Twenty patients
weretyped alsofor narcolepsy
DQB1*0603, which was negative in all patients. At the first visit
the mean body mass index (BMI) was 19.6 (SD 4.1; 95% CI 18.3–
20.9; median 19.2; range 14.0–35.3) kgm22. It increased by more
than 5% in 26/41 (65.4%) of the children. The main clinical
parameters are given in Table 1 and the Appendix S1 table.
To compare the clinical data of the children with onset in 2009
or 2010 (defined as POST) to those with an earlier onset (PRE) we
analyzed the data of the 65 children and adolescents aged less than
20 years at diagnosis in 2010. Seven of the 65 children had an
onset before 2009. The mean age at diagnosis was 15.3 years (95%
CI 11.8 to 18.7) in PRE and 12.1 y (11.2 to 13.0) in POST group
(P=0.0272). The time from onset of EDS to onset of cataplexy
was longer in the PRE (mean 546.2 days, 95% CI 48.7 to 1043.5)
group than in the POST (mean 35.9,days, 23.3 to 48.5;
P=0.0038) group. The time from onset of symptoms to diagnosis
was much longer in PRE (mean 47.6 months, 95% CI 14.2 to
80.9, range 17 to 111 months) than in POST group (mean 7.9,
95% CI 7.2 to 8.6, range 1 to 11 months; P,0.0001). There were
no statistically significant differences between the groups regarding
age at onset, cataplexy, aggressive behavior, or other symptoms
In 2010 we observed a significant increase in childhood
narcolepsy in Finland. In 2002–2009, the average annual
incidence of narcolepsy in children aged less than 17 years was
0.31 per 100 000 children. In 2010, the incidence rose to 5.3 per
100 000 children being about 17-fold higher as compared to
previous years. In adolescents aged 17 to19, the increase was
moderate (3-fold), and no increase was seen in adults $20 years of
age. Based on registry data, a significant (6.6-fold) increase in
childhood narcolepsy after Pandemrix vaccination has been
reported also from Sweden [36,37]. In a study from the Stockholm
area the small number of cases of narcolepsy (six among
vaccinated and two in the unvaccinated cohort) did not allow to
make reliable conclusions whether clearly increased incidence of
narcolepsy was found in that area . In the same study the
hazard ratio for Bell’s palsy among vaccinated against unvacci-
nated was 1.25 (1.06 to 1.48). Risk for Guillain-Barre ´ syndrome,
multiple sclerosis, type 1 diabetes, and rheumatoid arthritis
remained unchanged . There has been no evidence of
narcolepsy or other sleep-related adverse effects in recipients of
MF59-adjuvanted A(H1N1) pandemic or other MF59-adjuvanted
influenza vaccines .
In 2002 and 2009, narcolepsy was extremely rare in children
aged less than 11 years. Only one 9-year-old child had been
diagnosed in Finland in 2003. In 2010, twenty-two children aged
less than 11 years were diagnosed giving an annual incidence of
3.39/100 000 children in that age group. In the only previous
study of the incidence of narcolepsy, Silber and co-workers
presented the average annual incidence of narcolepsy to be 1.37/
100 000 and 0.74/100 000 for narcolepsy with cataplexy . The
annual incidence of narcolepsy in 2002–2009 (for all ages) in our
study is 0.79 (95% CI 0.62 to 0.96) corresponding well with the
published incidence rates .
All diagnoses were based on international criteria . The
symptoms were relatively severe (Table 1) but similar to those
described in Caucasian children in other recent publications
[34,40,41] and in the 1998–1999 series of 29 children from North
China . Psychiatric symptoms were common in our patients
(48%) as it was in other studies as well (Table 1) [3,40,41,42,43]. In
the Chinese study psychosocial problems were reported in 27/29
(93%) of children and academic problems were reported in 88% of
the cases . It is possible, however, that the psychiatric symptoms
are more severe in post-vaccination narcolepsy than in ‘‘normal
narcolepsy’’ prior to 2010. Four of our children have needed long-
term psychiatric hospitalisation and antipsychotic treatment was
needed in one. More studies are needed to study in more detail the
nature of psychiatric symptoms relative to type of narcolepsy. One
12-year-old boy had Type 1 diabetes. He was DQB1*0602 positive,
which is interesting since this HLA haplotype is regarded as
protective against type 1 diabetes. Knowing the strong genetic
association of narcolepsy with HLA type DQB1*0602 [3,7], it is
possible that all our narcoleptic children could be DQB1*0602
positive. Hypocretin-1 CSF levels were clearly below 110 pg/ml in
all 13 children who had their CSF specimen analysed.
Fatigue and sleepiness and also more severe neurological
complications have been associated with influenza [44,45].
Symptoms of the 1918 Spanish influenza consisted also of
excessive sleepiness having features similar to narcolepsy. The
present 2009 pandemic H1N1 virus is genetically and immuno-
logically more related with the Spanish influenza virus than recent
seasonal influenza viruses [35,46,47]. The present pandemic virus
has genes from avian, human and swine influenza viruses
[35,46,47,48] and the surface hemagglutinin (HA) and neuramin-
idase (NA) genes are more related to Spanish influenza genes than
to present seasonal influenza A virus genes . Unlike in the
Spanish flu, the present pandemic 2009 H1N1 virus usually causes
a mild infection and so far, except in North China , no
narcoleptic symptoms or increased incidence of narcolepsy has
been reported after the two epidemic seasons (2009/2010 and
2010/2011) of the present pandemic virus. It is anyway tempting
to hypothesize that the present cases of narcolepsy could have
been caused by an H1N1 influenza infection . However,
narcolepsy was not diagnosed in any of the nearly 8 000
laboratory confirmed H1N1 cases during the first pandemic wave
in 2009 . Also, none of our children had abnormal MRIs or
any signs of focal encephalitis/encephalopathy that has been
suggested to be involved in the destruction of the hypocretin cells
of the hypothalamic area .
Could the increased incidence be explained by the increased
awareness of narcolepsy in 2010 compared to previous years -
triggered by the intensive public discussion in Finland in August
2010 of the possible association of narcolepsy with Pandemrix
vaccination? This is unlikely since in the majority of our patients
the symptoms of sleepiness or cataplexy started abruptly before
August 2010, and the parents had consulted health care personnel
already during the winter or early spring 2010. At that time there
had been no news or articles of the increase in narcolepsy
incidence or its possible association with the H1N1 pandemic.
Also, the symptoms of narcolepsy and cataplexy were so clear that
they impaired the daily life of the children. It is very unlikely that
similarly severe abrupt sleepiness with cataplexy and behavioural
problems would have remained unnoticed before 2009–2010. In
addition, the incidence of adult and childhood narcolepsy in
Finland in 2002–2009 has been similar to that seen in other
The strength of this study is that it is based on nationwide
registries and thus includes the whole Finnish population. We are
Incidence and Symptoms of H1N1-Related Narcolepsy
PLoS ONE | www.plosone.org6March 2012 | Volume 7 | Issue 3 | e33723
confident that practically all severe narcoleptic children with a fast
onset in late 2009–2010 have been identified. Symptomatic
narcolepsy due to brain diseases was excluded by careful
neurological examination, EEG, MRI and other examinations
There are, however, certain clinical aspects that have to be
considered. In this study, we are limiting our analysis to the years
2002 to 2010. Some cases have remained undetected, since after
2010 we have continued to diagnose new children who have had
their first symptoms during the first half of 2010 (data not
presented). In most cases their initial symptoms have been less
severe than in the present series, explaining why they have not
been sent to specialists earlier. Thus our figure of post-Pandemrix
narcolepsy cases is an underestimation. This is consistent with the
reports showing that the delay from onset of symptoms to
narcolepsy diagnosis may be several years [3,43]. The increased
awareness of narcolepsy in Finland, starting in the fall 2010, has
lead to a clearly faster diagnosis. The incidence figures in previous
years are based on nationwide hospital discharge registry data.
Although the diagnostic criteria and practice in Finland has not
changed, the general awareness of narcolepsy has increased, which
may explain the slightly increased incidence figures in recent years.
To avoid bias we compared the incidence figures from 2009 and
2010 also from the same registries in a similar fashion for all years.
As stated above, the symptoms of most patients started during the
spring (median 38 days after vaccination; see table 1) before any
media attention took place. This means that the possible bias is
limited to the speed of diagnosis and not to the disease onset itself.
The onset, and the nature of symptoms and other diseases is based
on information from the patients, parents, school health nurses
and general practitioners, and some inadequacies are possible. In
two children EDS started on the day of vaccination. They both
fulfilled the diagnostic criteria of narcolepsy with cataplexy. Three
children had no clear cataplexy at the moment of diagnosis, but in
all of them the diagnosis was confirmed by the lack of CSF
hypocretin-1. One of them has developed cataplexy during the
follow-up about 18 months after onset of sleepiness. Four of the 50
post-vaccination patients were reported to suffer from ILI.
However, microbiological verification of influenza or any other
microbial infection had not been done suggesting a lack of
clinically important microbial infections in our narcoleptic
patients. Systematic determination of CSF hypocretin-1 and
HLA typing were not done for all patients since they are not
obligatory in the diagnosis when cataplexy is present and also
MSLT is verifying narcolepsy . Also some children/parents did
not give their consent for CSF specimen when it was suggested.
Narcolepsy is considered an immune-mediated, autoimmune
disease. In addition to a strong association especially with HLA
DQB1*0602/DRB1*15/DR15 – DQ6 HLA type, narcolepsy has
been associated with the presence of TRIB-2 antibodies ,
specific T-cell receptor alpha , and purinergic receptor
P2RY11  genotypes. The onset of symptoms in our children
has been very abrupt which contrasts with most other autoimmune
diseases  and with previous concepts of the natural course of
narcolepsy . The incidence of many autoimmune diseases has
increased over the past decades , but the rise has never been as
abrupt and as strong as in the case of childhood narcolepsy in
Finland in 2010.
What could be the mechanism(s) of a sudden increase in
childhood narcolepsy in Finland and Sweden? Co-occurrence is
not synonymous to causation. We have therefore carefully
differentiated the effects of the pandemic and the effects of the
vaccination. What is remarkable in Finland is that only the
Pandemrix vaccine was used and that the vaccination coverage
was very high in children and adolescents (75%). In the age-group
5 to 14 years the vaccination coverage was more than 80% .
In children the vaccination coverage in Sweden was approxi-
mately 67% [37,51]. In the case Pandemrix vaccination
contributed to the onset of narcolepsy, the high vaccination
coverage in Finland and Sweden may explain the highly increased
narcolepsy incidence in these countries as compared to those
countries where the H1N1 vaccination coverage was much lower,
e.g. in France 10% were vaccinated, and in Italy 0.3% . In the
Netherlands, where only few Pandemrix-related narcolepsy cases
were found, Pandemrix was used in healthy children aged 6
months to 5 years and in siblings and close relatives of children
aged less than 6 months, but not in older children . In addition
to the mathematical explanation, the lack of an increase in
narcolepsy in other countries would suggest that there are other
genetic or environmental factors, in addition to the AS03
adjuvanted vaccination, contributing to the onset. Recent
microbial infections have been suggested as possible environmen-
tal triggers that initiate the symptoms of narcolepsy . A likely
trigger in our patients could be influenza vaccination, which took
place in a close time-relation with the onset of narcolepsy.
Vaccination may have induced or accelerated already pre-existing
autoimmunity leading to a rapid destruction of the hypocretin cells
among genetically susceptible children and adolescents. All our
HLA typed patients (n=32) were of HLA DQB1*0602 type,
which is the presently known major genetic susceptibility factor
. We cannot formally rule out the contribution of other
infectious agents (H1N1, seasonal influenza, enterovirus, rhinovi-
rus, streptococcal infection , or some other microbial
infections) together with vaccination that could have lead to the
development of narcolepsy.
A recent Chinese study observed an epidemiological link with
pandemic influenza and narcolepsy, without significant relation to
vaccination, showing a 3-fold increased incidence of narcolepsy 5
to 7 months after the 2009 epidemics . ILI, without
microbiological confirmation, was observed in only 4 of our
patients (10%). There was, thus, no evidence of simultaneous or
closely time-related microbial infections in most of our patients.
Unfortunately, serum or CSF specimens for detailed viral and
streptococcal antibody analyses were not available since there was
no suspicion of microbial etiology of the narcoleptic symptoms.
Almost a similar increase in the incidence of narcolepsy has been
reported in Sweden, where only Pandemrix was used, as in
Finland [36,37]. Although isolated cases of narcolepsy have been
diagnosed also after other vaccinations than Pandemrix, there is
no evidence of an increased risk of narcolepsy with any other
vaccine than the AS03 adjuvanted Pandemrix [34,39,55]. In order
to draw a link between an environmental effect one must show
that the incidence has increased significantly as compared to
previous years. This was the case in the Chinese study, where the
increase was not explained by Pandemrix . Combining their
and our results, it seems that several different or multiple triggering
factors may exist for the narcolepsy to develop. Based on our study
and the study by Nohynek and co-workers  Pandemrix vaccine
was likely one of the triggers. The role of possible other closely
time-related triggers remains to be studied in future epidemiolog-
ical and experimental studies.
The adjuvant (AS03) in the Pandemrix vaccine is very potent,
since it frequently induces local inflammatory reactions and
occasional systemic side effects like fever. We can speculate that
the inflammatory response was so strong that it included central
nervous system affection. It is, however, difficult to say whether the
systemic response produced by the adjuvant could have caused
cellular damage. The hypocretin-1 levels were very low or
Incidence and Symptoms of H1N1-Related Narcolepsy
PLoS ONE | www.plosone.org7 March 2012 | Volume 7 | Issue 3 | e33723
undetectable in our narcoleptic patients, which may indicate a
rapid destruction of hypocretin cells within weeks or a few months
after vaccination. This does not, however, mean that the adjuvant
would be causing hypocretin cell damage directly. Rather, there is
a possibility that already an ongoing autoimmune process was
accelerated by the nonspecific inflammatory responses induced by
the vaccine or its specific components leading to the destruction of
hypocretin producing cells. Thus, theoretically any inflammatory
process whether it is iatrogenic or infectious (such as influenza and
streptococcal infection) could non-specifically enhance the auto-
immune process leading to narcolepsy. There is also a possibility
that some other genetic factors, in addition to the commonly
accepted HLA-DAB1*0602 risk allele [8,11,16,23,56] are en-
riched in Finland and Sweden making Nordic children susceptible
to an acute inflammation/autoimmune-related narcolepsy.
Why were adult onset cases not increased? Theoretically it is
possible that the vaccine precipitated onset in people who would
have developed it later, anyway. In this case we should see a drop
in adult incidence later during the coming years. Another
possibility is that some children with multiple genetic predisposi-
tion factors are especially vulnerable to develop narcolepsy. In
some other autoimmune diseases, such as in type 1 diabetes, early
age onset is also often seen. It is also possible that the onset is more
insidious in older adolescents and adults and thus there may be a
delay in the diagnosis. In this case we would expect an increase in
the incidence in adults during the coming years. All this mandates
continued clinical and epidemiological surveillance in the future.
We observed a 17-fold increase in the annual incidence of
narcolepsy in 2010 as compared to previous years in children aged
under 17 years of age. A common feature in the history of our 54
newly diagnosed childhood narcoleptic patients was that 50
children had received an adjuvanted pandemic influenza vaccine
(Pandemrix) within 8 months before the onset of symptoms. In
most cases, the development of symptoms was fast. We consider it
likely that Pandemrix vaccination contributed to the increased
incidence of narcolepsy in Finland in 2010 in HLA DQB1*0602
positive children. Our observations warrant further studies on the
role of different environmental factors as well as pathogenetic
studies to understand how a vaccination/adjuvant and other
environmental triggers can cause narcolepsy.
narcolepsy (, ,17 years of age) diagnosed in 2010 in
Clinical characteristics of 54 children with
2010 in Finland in different age groups. The highest peak
was seen in children aged 11 to 13 years of age. No children aged
less than 8 years had been diagnosed in Finland before 2010.
Occurrence of childhood narcolepsy in 2002–
age-groups. The highest incidences were seen in children aged
from 11 to 16 years – especially in the age-group 11 to 13 years of
Incidence of narcolepsy by year in different
We thank all our colleagues and everybody else who have been involved in
the diagnosis and treatment of these children and adolescents.
Conceived and designed the experiments: MP OSH CH TK IJ. Performed
the experiments: MP OSH TK IJ II ML PO PN RA TW ME HR JO HS
HA PK. Analyzed the data: MP OSH TK IJ II ML PO PN RA TW ME
HR JO HS HA PK. Contributed reagents/materials/analysis tools: MP IJ
ML PK. Wrote the paper: MP TK IJ OSH CH. Provided input on the
manuscript: II PO PN RA TW ME HR JO HS HA PK.
1. Hublin C, Kaprio J, Partinen M, Koskenvuo M, Heikkila ¨ K, et al. (1994) The
prevalence of narcolepsy: an epidemiological study of the Finnish Twin Cohort.
Ann Neurol 35: 709–716.
Silber MH, Krahn LE, Olson EJ, Pankratz VS (2002) The epidemiology of
narcolepsy in Olmsted County, Minnesota: a population-based study. Sleep 25:
Dauvilliers Y, Arnulf I, Mignot E (2007) Narcolepsy with cataplexy. Lancet 369:
American Academy of Sleep Medicine (2005) The International classification of
sleep disorders, 2nd edition. Westchester, Ill: American Academy of Sleep
Longstreth WT, Jr., Koepsell TD, Ton TG, Hendrickson AF, van Belle G (2007)
The epidemiology of narcolepsy. Sleep 30: 13–26.
Mignot E, Lammers GJ, Ripley B, Okun M, Nevsimalova S, et al. (2002) The
role of cerebrospinal fluid hypocretin measurement in the diagnosis of
narcolepsy and other hypersomnias. Arch Neurol 59: 1553–1562.
Mignot E, Lin L, Rogers W, Honda Y, Qiu X, et al. (2001) Complex HLA-DR
and -DQ interactions confer risk of narcolepsy-cataplexy in three ethnic groups.
Am J Hum Genet 68: 686–699.
Hor H, Kutalik Z, Dauvilliers Y, Valsesia A, Lammers GJ, et al. (2010) Genome-
wide association study identifies new HLA class II haplotypes strongly protective
against narcolepsy. Nat Genet 42: 786–789.
Knudsen S, Jennum PJ, Alving J, Sheikh SP, Gammeltoft S (2010) Validation of
the ICSD-2 criteria for CSF hypocretin-1 measurements in the diagnosis of
narcolepsy in the Danish population. Sleep 33: 169–176.
10. Tanaka S, Honda Y, Honda M (2008) MX2 gene expression tends to be
downregulated in subjects with HLA-DQB1*0602. Sleep 31: 749–751.
11. Hallmayer J, Faraco J, Lin L, Hesselson S, Winkelmann J, et al. (2009)
Narcolepsy is strongly associated with the T-cell receptor alpha locus. Nat Genet
12. Honda M, Eriksson KS, Zhang S, Tanaka S, Lin L, et al. (2009) IGFBP3
colocalizes with and regulates hypocretin (orexin). PLoS ONE 4: e4254.
13. Miyagawa T, Honda M, Kawashima M, Shimada M, Tanaka S, et al. (2010)
Polymorphism located in TCRA locus confers susceptibility to essential
hypersomnia with HLA-DRB1*1501-DQB1*0602 haplotype. J Hum Genet
14. Shimada M, Miyagawa T, Kawashima M, Tanaka S, Honda Y, et al. (2010) An
approach based on a genome-wide association study reveals candidate loci for
narcolepsy. Hum Genet 128: 433–441.
15. Han F, Lin L, Li J, Aran A, Dong SX, et al. (2012) TCRA, P2RY11, and
CPT1B/CHKB associations in Chinese narcolepsy. Sleep Medicine 13(3):
16. Kornum BR, Kawashima M, Faraco J, Lin L, Rico TJ, et al. (2011) Common
variants in P2RY11 are associated with narcolepsy. Nat Genet 43: 66–71.
17. Cvetkovic-Lopes V, Bayer L, Dorsaz S, Maret S, Pradervand S, et al. (2010)
Elevated Tribbles homolog 2-specific antibody levels in narcolepsy patients.
J Clin Invest 120: 713–719.
18. Kawashima M, Lin L, Tanaka S, Jennum P, Knudsen S, et al. (2010) Anti-
Tribbles homolog 2 (TRIB2) autoantibodies in narcolepsy are associated with
recent onset of cataplexy. Sleep 33: 869–874.
19. Toyoda H, Tanaka S, Miyagawa T, Honda Y, Tokunaga K, et al. (2010) Anti-
Tribbles homolog 2 autoantibodies in Japanese patients with narcolepsy. Sleep
20. Tanaka S, Honda M (2010) IgG abnormality in narcolepsy and idiopathic
hypersomnia. PLoS ONE 5: e9555.
21. Santos Coelho FM, Pradella-Hallinan M, Pedrazzoli M, Soares CAS,
Fernandes GBP, et al. (2011) Low CD40L levels and relative lymphopenia in
narcoleptic patients. Human immunology 72: 817–820.
22. Overeem S, Black JL, Lammers GJ (2008) Narcolepsy: Immunological aspects.
Sleep Medicine Reviews 12: 95–107.
23. Fontana A, Gast H, Reith W, Recher M, Birchler T, et al. (2010) Narcolepsy:
Autoimmunity, effector T cell activation due to infection, or T cell independent,
major histocompatibility complex class II induced neuronal loss. Brain 133:
Incidence and Symptoms of H1N1-Related Narcolepsy
PLoS ONE | www.plosone.org8 March 2012 | Volume 7 | Issue 3 | e33723
24. Kornum BR, Faraco J, Mignot E (2011) Narcolepsy with hypocretin/orexin Download full-text
deficiency, infections and autoimmunity of the brain. Curr Opin Neurobiol 21:
25. Han F, Lin L, Warby SC, Faraco J, Li J, et al. (2011) Narcolepsy onset is
seasonal and increased following the 2009 H1N1 pandemic in china. Ann
Neurol 70: 410–417.
26. Aran A, Lin L, Nevsimalova S, Plazzi G, Hong SC, et al. (2009) Elevated anti-
streptococcal antibodies in patients with recent narcolepsy onset. Sleep 32:
27. Longstreth WT, Jr., Ton TGN, Koepsell TD (2009) Narcolepsy and
streptococcal infections. Sleep 32: 1548.
28. Talk about sleep (2010) Possible Link Between H1N1 and Narcolepsy. Available:
cessed 2012 Feb 23.
29. Medical Products Agency (Lakemedelsverket) (2010) The MPA investigates
reports of narcolepsy in patients vaccinated with Pandemrix. Available: http://
cessed 2012 Feb 23.
30. WHO (2010) Pandemrix vaccine and cases of narcolepsy. Available: http://
en/. Accessed 2012 Feb 23.
31. THL (2010) National Institute for Health and Welfare recommends discontin-
uation of Pandemrix vaccinations. Finnish National Institute for Health and
Welfare website. Available: http://wwwthlfi/en_US/web/en/pressrelease?id=
22930. Accessed 2012 Feb 23.
32. THL (2011) Association between Pandemrix and narcolepsy confirmed among
Finnish children and adolescents. Finnish National Institute for Health and
Welfare website. Available: http://wwwthlfi/en_US/web/en/pressrelease?id=
26352. Accessed 2012 Feb 23.
33. Nohynek H, Jokinen J, Partinen M, Vaarala O, Kirjavainen T, et al. (2012)
AS03 adjuvanted AH1N1 vaccine associated with an abrupt increase in the
incidence of childhood narcolepsy in Finland. PLoS ONE. In press.
34. Dauvilliers Y, Montplaisir J, Cochen V, Desautels A, Einen M, et al. (2010) Post-
H1N1 narcolepsy-cataplexy. Sleep 33: 1428–1430.
35. Ikonen N, Haanpaa M, Ronkko E, Lyytikainen O, Kuusi M, et al. (2010)
Genetic diversity of the 2009 pandemic influenza A(H1N1) viruses in Finland.
PLoS ONE 5: e13329.
36. Medical Products Agency (Lakemedelsverket) (2011) A registry based compar-
ative cohort study in four Swedish counties of the risk for narcolepsy after
vaccination with Pandemrix - A first and preliminary report, by the Medical
Products Agency. Available: http://wwwlakemedelsverketse/upload/nyheter/
2011/PandemrixRegReport110328pdf. Accessed 2012 Feb 23. pp 1–10.
37. Medical Products Agency (Lakemedelsverket) (2011) Occurrence of narcolepsy
with cataplexy among children and adolescents in relation to the H1N1
pandemic and Pandemrix vaccinations - Results of a case inventory study by the
MPA in Sweden during 2009-2010. Available: http://wwwlakemedelsverketse/
cessed 2012 Feb 23.
38. Bardage C, Persson I, Ortqvist A, Bergman U, Ludvigsson JF, et al. (2011)
Neurological and autoimmune disorders after vaccination against pandemic
influenza A (H1N1) with a monovalent adjuvanted vaccine: population based
cohort study in Stockholm, Sweden. BMJ 343: d5956.
39. Tsai TF, Crucitti A, Nacci P, Nicolay U, Cioppa GD, et al. (2011) Explorations
of clinical trials and pharmacovigilance databases of MF59((R))-adjuvanted
influenza vaccines for associated cases of narcolepsy. Scand J Infect Dis 43(9):
40. Aran A, Einen M, Lin L, Plazzi G, Nishino S, et al. (2010) Clinical and
therapeutic aspects of childhood narcolepsy-cataplexy: a retrospective study of
51 children. Sleep 33: 1457–1464.
41. Nevsimalova S, Jara C, Prihodova I, Kemlink D, Sonka K, et al. (2011) Clinical
features of childhood narcolepsy. Can cataplexy be foretold? Eur J Paediatr
Neurol 15: 320–325.
42. Han F, Chen E, Wei H, Dong X, He Q, et al. (2001) Childhood narcolepsy in
North China. Sleep 24: 321–324.
43. Guilleminault C, Cao MT (2011) Narcolepsy: Diagnosis and management. In:
Kryger MH, Roth T, Dement WC, eds. Principles and practice of sleep
medicine. 5th edition. St. Louis, MO: Elsevier Saunders. pp 957–968.
44. Webster RI, Hazelton B, Suleiman J, Macartney K, Kesson A, et al. (2010)
Severe encephalopathy with swine origin influenza A H1N1 infection in
childhood: case reports. Neurology 74: 1077–1078.
45. Montastruc JL, Durrieu G, Rascol O (2011) Pandemrix, (H1N1)v influenza and
reported cases of narcolepsy. Vaccine 29: 2010.
46. Hancock K, Veguilla V, Lu X, Zhong W, Butler EN, et al. (2009) Cross-reactive
antibody responses to the 2009 pandemic H1N1 influenza virus. N Engl J Med
47. Itoh Y, Shinya K, Kiso M, Watanabe T, Sakoda Y, et al. (2009) In vitro and in
vivo characterization of new swine-origin H1N1 influenza viruses. Nature 460:
48. Miller E, Hoschler K, Hardelid P, Stanford E, Andrews N, et al. (2010)
Incidence of 2009 pandemic influenza A H1N1 infection in England: a cross-
sectional serological study. Lancet 375: 1100–1108.
49. Nishino S, Kanbayashi T (2005) Symptomatic narcolepsy, cataplexy and
hypersomnia, and their implications in the hypothalamic hypocretin/orexin
system. Sleep Med Rev 9: 269–310.
50. Bach JF (2002) The effect of infections on susceptibility to autoimmune and
allergic diseases. N Engl J Med 347: 911–920.
51. Mereckiene J, Cotter S, Weber J, Nicoll A, D’Ancona F, et al. (2012) Influenza
A(H1N1)pdm09 vaccination policies and coverage in Europe. Euro Surveill
52. Lareb (2011) Overview of Dutch cases of narcolepsy associated with Pandemic
influenza vaccine (PandemrixH). Lareb website. Available: http://wwwlarebnl/
2012 Feb 23.
53. Picchioni D, Hope CR, Harsh JR (2007) A case-control study of the
environmental risk factors for narcolepsy. Neuroepidemiology 29: 185–192.
54. Kraemer HC, Stice E, Kazdin A, Offord D, Kupfer D (2001) How do risk
factors work together? Mediators, moderators, and independent, overlapping,
and proxy risk factors. Am J Psychiatry 158: 848–856.
55. Wijnans L, de Bie S, Dieleman J, Bonhoeffer J, Sturkenboom M (2011) Safety of
pandemic H1N1 vaccines in children and adolescents. Vaccine 29: 7559–7571.
56. Deloumeau A, Bayard S, Coquerel Q, Dechelotte P, Bole-Feysot C, et al. (2010)
Increased immune complexes of hypocretin autoantibodies in narcolepsy. PLoS
ONE 5: e13320.
Incidence and Symptoms of H1N1-Related Narcolepsy
PLoS ONE | www.plosone.org9 March 2012 | Volume 7 | Issue 3 | e33723