Vaccine (2008) 26, 1786—1793
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/vaccine
Influenza vaccination of recommended adult
populations, U.S., 1989—2005?
Pengjun Lua,∗, Carolyn B. Bridgesb,
Gary L. Eulera, James A. Singletona
aImmunization Services Division, National Center for Immunization and Respiratory Diseases,
Centers for Disease Control and Prevention, 1600 Clifton Road, NE, Atlanta, GA 30333, United States
bInfluenza Division, National Center for Immunization and Respiratory Diseases,
Centers for Disease Control and Prevention, 1600 Clifton Road, NE, Atlanta, GA 30333, United States
Received 15 October 2007; received in revised form 12 December 2007; accepted 3 January 2008
Available online 14 February 2008
Objective: To assess influenza vaccination coverage among recommended adult populations in
the United States.
Methods: Data from the 1989 to 2005 National Health Interview Surveys (NHISs), weighted to
reflect the civilian, non-institutionalized U.S. population, were analyzed to determine self-
reported levels of influenza vaccination among persons aged ≥65 years, persons with high-risk
conditions, health care workers (HCW), pregnant women, and persons living in households with
at least one identified person at high risk of complications from influenza infection. We stratified
data by race/ethnicity to identify racial/ethnic disparities.
Results: Vaccination coverage levels among all recommended adult populations peaked in 2004,
then declined in 2005 in association with the 2004—2005 vaccine shortage. Coverage for adults
≥65 years of age increased from 30.1% (95% confidence interval [CI]: 28.8—31.3) in 1989 to 70.0%
(68.0—71.5) in 2004. In 2004, coverage was 40.7% (39.0—42.5) for all adults 50—64 years, 27.2%
(24.6—29.9) for adults aged 18—49 years with high-risk conditions, 43.2% (39.9—46.6) for health
care workers, 21.1% (19.1—23.4) for non-high-risk adults aged 18—64 years with a high-risk
household member, and 14.4% (8.8—22.9) for pregnant women. Among each of the recom-
mended adult sub-groups, vaccination coverage was higher for non-Hispanic whites compared
to minority groups.
?Disclaimer:The findings and conclusions in this presentation are those of the authors and do not necessarily represent the views of CDC.
∗Corresponding author at: Immunization Services Division, National Center for Immunization and Respiratory Diseases, Centers for
Disease Control and Prevention, 1600 Clifton Road, NE, Mail Stop E-62, Atlanta, GA 30333, United States.. Tel.: +1 4046398844;
fax: +1 4046393266.
E-mail address: email@example.com (P. Lu).
0264-410X/$ — see front matter. Published by Elsevier Ltd.
Influenza vaccination of recommended adult populations1787
Conclusions: By 1997, influenza vaccination coverage had exceeded the national 2000 objective
of 60% among persons aged ≥65 years, but by 2004 still remains well below the national 2010
target of 90%. Coverage levels for other groups targeted for influenza vaccination also are far
short of the Healthy People 2000 and 2010 goals of 60% for persons aged 18—64 years with high-risk
conditions, health care workers, and pregnant women. A concerted effort to increase provider
adoption of standards for adult immunization, public awareness, and stable vaccine supplies are
needed to improve influenza vaccination rates among recommended groups, and to reduce racial
and ethnic disparities.
Published by Elsevier Ltd.
Influenza is a major cause of morbidity and mortality
among adults in the United States. During the 1990—1999
influenza seasons, approximately 36,000 excess deaths were
attributed annually to influenza with more than 90% of
deaths occurring in persons 65 years and older [1,2].
Influenza-related disease is responsible for an average of
226,000 excess hospitalizations per year in the United
States . The influenza vaccine has been proven to be an
efficacious and cost-effective tool for reducing morbidity
and mortality associated with influenza in adults [4—11].
Influenza vaccination of healthy working adults <65 years
of age can decrease the rates of influenza-like illness, lost
workdays, and physician visits and can prevent illness and
complications among children and older adults [11—16]. The
economic impact of influenza infection is substantial. The
cost of a severe influenza epidemic has been estimated to be
$87.1 billion . Much of this illness, death and associated
economic costs could be prevented with higher influenza
vaccination coverage levels.
Influenza vaccination is recommended for persons at
increased risk for complications from influenza, includ-
ing all persons aged 65 years and older, younger persons
with chronic diseases such as diabetes, chronic heart con-
ditions, chronic obstructive pulmonary disease (COPD),
asthma, cancer, and kidney failure, and women who will
be pregnant during the influenza season. Influenza vac-
cine is also recommended for persons who can transmit
influenza to those at high risk such as health care workers
(HCW), and household contacts . In addition, all per-
sons 50—64 years are recommended to be vaccinated as
a substantial proportion of persons in this group have one
or more medical conditions which increase their risk of
Despite the presence of safe and effective vaccine and
long-standing recommendations to provide annual influenza
vaccination to target populations, vaccination levels are
suboptimal [18—26]. To assess progress toward achieving
2010 national health objectives and implementation of the
Advisory Committee on Immunization Practice (ACIP) rec-
ommendations, we analyzed the data from 1989 to 2005
National Health Interview Survey (NHIS). This study eval-
uated: (1) influenza vaccination prevalence among persons
aged ≥65 years, persons 50—64 years, adults aged <65 years
with high-risk conditions, health care workers, pregnant
women, and persons in close contact with persons at high
risk; (2) trends in vaccination; and (3) racial/ethnic dispar-
ities in influenza vaccination among target populations.
The National Health Interview Survey is a national household
survey conducted annually by National Center for Health
since 1957. The NHIS questionnaire consists of two basic
parts: a set of basic health and demographic items; and
one or more sets of questions on current health topics .
This analysis used variables from the sample adult core. In
the sample adult core, one adult per sampled family was
randomly selected and asked to complete the sample adult
questionnaire. In 2004, a total of 31,326 adults aged 18
years and older participated. The final response rate for the
sample adult core was 72.5%.
Participants were asked ‘‘During the past 12 months,
have you had a flu shot?’’ Vaccination status was deter-
mined by their affirmative or negative answer. Respondents
were also asked whether they had ever been told by
a doctor or other health professional that they had
‘‘emphysema,’’ ‘‘chronic bronchitis,’’ ‘‘coronary heart
disease,’’ ‘‘angina,’’ ‘‘a heart attack,’’ any other heart
condition or heart disease,’’ ‘‘diabetes,’’ ‘‘kidney fail-
ure,’’ or ‘‘cancer’’. Questions regarding asthma status
were ‘‘Have you ever been told by a doctor or other health
professional that you had asthma?’’ and ‘‘During the past
12 months, have you had an episode of asthma or asthma
attack?’’ In addition, female respondents were asked ‘‘Are
you currently pregnant?’’
We defined high-risk persons as individuals who self-
reported one or more of the following: ever being told by
a physician they had diabetes, emphysema, coronary heart
disease, angina, heart attack or other heart condition; being
diagnosed with cancer in the past 12 months (excluding non-
melanoma skin cancer) or ever being told by a physician
they have lymphoma, leukemia or blood cancer; during the
past 12 months, being told by a physician they have chronic
bronchitis or weak or failing kidneys; or reporting an asthma
episode or attack in the past 12 months.
Individuals were classified as health care workers if
they were currently employed in a health care occu-
pation or in a health care industry setting, based on
standard occupation and industry categories recoded into
categories by the National Center for Health Statistics
at CDC. Health occupation included health diagnosing
occupations, health assessment and treating occupations,
health technologists, and health service occupations.
Health care settings included hospitals, nursing or per-
sonal care facilities, and offices of physicians, dentists,
chiropractors, optometrists or other health care practition-
1788 P. Lu et al.
ers. Classification of HCW is described further in Walker
et al. .
Household contacts were defined as persons aged 18—64,
who themselves were not in an high-risk group and not an
HCW, living in households with at least one identified per-
son at high risk of complications from influenza infection.
Because only one sample adult is selected per family, high-
risk adult family members could only be identified on the
basis of age at the time of the survey (≥65 years). High-
risk child family members were identified based on age and
high-risk conditions (all children <2 years, and children 2—17
years with high-risk conditions such as asthmas, cystic fibro-
sis, sickle cell anemia, diabetes, congenital heart disease,
and other heart conditions). In addition, since 1997 a sam-
ple child/adult was selected for each family, not just each
We assessed influenza vaccination among ACIP adult tar-
get groups and stratified by race/ethnicity. Wald Chi-square
statistical tests were performed to see if racial/ethnic dif-
ferences within each target group were significant. Influenza
vaccination rates from 1989 to 2005 or 1997 to 2005 were
evaluated based on whether the variables were available or
not in each survey year. A test for linear trends in recent
years was conducted for each target group . Individuals
who refused to answer the influenza vaccination question or
did not know their vaccination status were excluded from
the analysis. There were 1.4% (275) individuals who did not
know their influenza vaccination status in 2004. This pro-
portion varied from 1989 to 2005 (ranged from 0.01% to
2.0%). Since 2004, a question on receipt of the influenza
nasal spray vaccination was included in the questionnaire,
but we excluded those data from the analysis because the
number who reported vaccination with the spray alone in
the past 12 months was very small (<0.5%) and these reports
may be less accurate. To better approximate past season
coverage, we reported coverage restricted to individuals
interviewed during February through August. For exam-
ple, respondents interviewed February—August 2004 were
analyzed to estimate influenza vaccine coverage for the
2003—2004 influenza season. We focused primarily on NHIS
2004 data (2003—2004 season) in the results because of a
vaccine shortage for the 2004—2005 season, which was asso-
ciated with substantially lower vaccine coverage estimates
based on the 2005 NHIS data.
SUDAAN statistical software (Software for the statisti-
cal analysis of correlated data, Research Triangle Institute;
Research Triangle Park, NC) was used to generate point esti-
mates and 95% confidence intervals and to account for the
complex sampling design of the National Health Interview
Survey. All analyses were weighted to reflect the age, sex,
and race/ethnicity of the U.S. non-institutionalized civilian
variables, 18,356 participants were included in the analysis
in 2004; among those adults, 22.4% were 50—64 years of
age, and 19.2% were 65 years or older. Among persons aged
18—64 years, 18.7% reported having a high-risk condition.
Health care workers made up 8.2% of respondents.
1989—2005. *Each year includes persons interviewed dur-
ing February—August of the year and approximates coverage
for the prior fall-winter vaccination period (2005 means
2004—2005 season, and so on). **Household contacts are
persons aged 18—64 years living in households with at least one
identified person at high risk of complications from influenza
infection including children <2 years. Vaccination of contacts
of children <2 years was just encouraged for 2002—2003 and
2003—2004 seasons, and then fully recommended for the
2004—2005 season of vaccine shortage.
Influenza vaccination coverage in 2004 for adults ≥65
years of age was 70.0% (95% CI=68.0—71.5%) compared
to 40.7% (95% CI=39.0—42.5%) for persons aged 50—64
years. During 1989—1999, influenza vaccination coverage
for elderly adults increased steadily each year (test for
trend, p<0.05), but plateaued from 1999 to 2004 (test for
trend p>0.05) (Fig. 1, Table 1). The coverage dropped sub-
stantially in 2005. In 2004, vaccination coverage levels for
persons ≥65 years of age were 72.7% (95% CI=70.9—74.4%)
for non-Hispanic whites, 49.5% (95% CI=43.1—55.9%) for
non-Hispanic blacks, and 59.0% (95% CI=52.5—65.3%) for
Hispanics (Table 2).
In 2004, coverage among persons aged 18—64 years
with high-risk conditions was lower for younger adults
(ages 18—49: 27.2% [95% CI=24.6—29.9%] vs. ages 50—64:
50.5% [95% CI=47.2—53.9%]) (Table 1) overall and in each
racial/ethnic group (Table 2). In both age groups (18—49 and
50—64), vaccination coverage was higher among high-risk
persons (27.2% and 50.5%, respectively) compared to those
without identified high-risk conditions (18.6% and 36.8%,
respectively). From 1999 to 2004, influenza vaccination
coverage among those aged 50—64 years with high-risk con-
ditions remained at a stable level (test for trend: p>0.05)
(Fig. 1, Table 1), but coverage slowly increased for persons
aged 18—49 years with high-risk conditions during the same
period (test for trend, p<0.05). Among high-risk persons in
both age groups (18—49 and 50—64), coverage substantially
declined in 2005. Vaccination coverage in both age groups
was significantly lower for non-Hispanic blacks and Hispanics
compared to non-Hispanic whites (p<0.01) (Table 2).
Influenza vaccination of recommended adult populations
Influenza vaccination coverage for recommended adult populations, National Health Interview Survey (NHIS)a
NHIS 2004NHIS 2005
Sample size Weighted sample size Vaccination rate (95% CI)Sample size Weighted sample size Vaccination rate (95% CI)
40.7 (39.0, 42.5)
70.0 (68.0, 71.5)
21.4 (20.1, 22.8)
60.2 (58.3, 62.2)
Persons with high-risk condition
37.5 (35.3, 39.8)
27.2 (24.6, 29.9)
50.5 (47.2, 53.9)
24.5 (22.8, 26.3)
17.6 (15.5, 19.9)
33.0 (30.3, 35.7)
Persons without high-risk condition
22.8 (21.8, 23.7)
18.6 (17.6, 19.6)
36.8 (34.8, 38.9)
14.4 (8.8, 22.9)
43.2 (39.9, 46.6)
10.3 (9.7, 10.9)
8.5 (7.9, 9.2)
16.2 (14.6, 17.8)
13.2 (8.3, 20.3)
32.8 (30.1, 35.5)
Household contacts of persons at high risk including children <2 years
21.1 (19.1, 23.4)
17.4 (15.4, 19.5)
38.1 (32.3, 44.3)
10.3 (8.7, 12.0)
7.5 (6.0, 9.2)
22.4 (17.7, 27.8)
aThis table is based on February—August interviews only.
1790P. Lu et al.
Influenza vaccination coverage among recommended adult populations by race/ethnicity (2004 NHIS)a
Vaccination rate (95% CI)
NH-whiteNH-black Hispanic Others
43.7 (41.6, 45.8)
72.7 (70.9, 74.4)
27.3 (23.2, 31.9)
49.5 (43.1, 55.9)
32.0 (27.4, 37.1)
59.0 (52.5, 65.3)
36.6 (28.1, 46.0)
61.8 (47.0, 74.6)
Persons with high-risk condition
39.6 (37.0, 42.3)
28.3 (25.1, 31.7)
52.9 (48.9, 56.8)
18.9 (10.9, 30.8)
47.3 (43.3, 51.5)
28.9 (24.1, 34.1)
24.1 (18.3, 31.0)
36.4 (28.2, 45.4)
29.1 (24.0, 34.8)
20.1 (14.5, 27.3)
45.0 (36.7, 53.5)
50.6 (40.3, 60.8)
42.0 (27.1, 58.4)
59.7 (43.0, 74.4)
29.4 (22.5, 37.4)33.2 (25.0, 42.4) 46.1 (30.5, 62.5) <0.01
Household contacts of persons at high risk including children <2 years
18—64 26.5 (23.6, 29.7)
18—4922.0 (19.1, 25.2)
50—64 41.8 (35.0, 48.9)
12.8 (8.2, 19.3)
11.7 (7.1, 18.7)
11.0 (8.1, 14.7)
9.6 (6.9, 13.3)
15.1 (8.4, 25.5)
13.5 (7.0, 24.3)
aThis table is based on February—August interviews only. NH stands for non-hispanic.
bThe standard of reliability is RSE<0.3 (where RSE=the ratio of the standard error and the prevalence, se/percent), this estimate did
not meet the standard or there were <30 respondents in the denominator.
***p value indicates chi-square test for association between racial/ethnic groups with NH-white as the reference group.
In 2004, 4.4% (156) women aged 18—44 years reported
they were pregnant when interviewed. From 1997 to 2004,
vaccination coverage among pregnant women aged 18—44
years without any high-risk conditions ranged from a low of
9.3% in 2002 to 14.4% in 2004 (Fig. 1, Table 1) and did not
differ statistically compared to non-pregnant women of the
same age group (p>0.05) (data not shown). Coverage among
pregnant women was stable during 1997—2004 (test for
trend, p>0.05). Influenza vaccination coverage was 18.9%
in 2004 for non-Hispanic white women who were currently
pregnant with no reported medical conditions; estimates for
other racial and ethnic groups were unreliable due to small
numbers (Table 2).
Among healthcare workers,
increased from 8.3% (95% CI=7.4—9.4%) in 1989 to 43.2%
(95% CI=39.9—46.6%) in 2004 (test for trend, p<0.05)
(Fig. 1, Table 1), but was significantly lower in 2005. Non-
Hispanic white HCWs reported significantly higher coverage
(47.3%, 95% CI=43.3—51.5%) than non-Hispanic African-
Americans (29.4%, 95% CI=22.5—37.4%), and Hispanics
(33.2%, 95% CI=25.0—42.4%) (Table 2).
Among identified household contacts aged 18—64 years
who lived with a high-risk person, influenza vaccination cov-
erage ranged from 15.7% to 21.1% during 2002—2004 and was
significantly lower in 2005 (Fig. 1). Household contacts aged
18—49 years were significantly less likely than persons aged
18—49 with high-risk conditions to report an influenza vacci-
29.9%, respectively); a similar pattern was observed for
persons aged 50—64 years (Table 1). Among household
contacts aged 18—64 years, the estimated percentages of
non-Hispanic blacks and Hispanics having received influenza
vaccination (12.8%, 95% CI=8.2—19.3% and 11.0%, 95%
CI=8.1—14.7%, respectively) were lower than those for
non-Hispanic whites (26.5%, 95% CI=23.6—29.7%). These
racial/ethnic disparities remained when age groups were
split into 18—49 years and 50—64 years (Table 2).
In 2005, vaccination coverage among each of the recom-
mended adult sub-groups was significantly lower compared
to 2004, except for pregnant women (Fig. 1, Table 1).
The results of this study indicate that national influenza vac-
cination coverage among persons aged ≥65 years of age sub-
after 1999. A significant drop occurred in 2005 due to the
2004—2005 influenza vaccine shortage. Among adults aged
≥65 years, the 2000 national health objective level of 60%
influenza vaccination coverage was first attained in 1997.
However, looking toward year 2010, influenza vaccination
coverage in 2004 (70.0%) still remained well below the 90%
unless effective intervention strategies are developed and
programs are widely implemented to dramatically improve
influenza vaccination coverage among this age group.
Influenza vaccination levels were also far below target
levels for adults 18—64 years old with high-risk conditions
and were significantly affected by delays and shortages in
influenza vaccine. The substantial and unexpected reduc-
tion in the supply of influenza vaccine for the 2004—2005
season and the delay in vaccine distribution in 2000—2001
are reflected in the coverage results [29,30], but do not
explain all of the lack of recent progress in improving
influenza vaccine coverage in adults aged ≥65 years and
high-risk persons aged 18—64 years.
Although substantial data exists regarding the benefits of
influenza vaccine in high-risk groups, only one randomized
trial of influenza vaccine has been conducted among com-
recent articles have questioned the benefit of the influenza
vaccine particularly among the elderly [34—36]. Although
many frail elderly may not respond optimally to influenza
Influenza vaccination of recommended adult populations 1791
vaccination, the vaccine remains the single best preven-
tion tool against influenza and its complications. Raising
influenza vaccination rates among the elderly and among
their contacts, including health care workers, can only serve
to better protect those most vulnerable to influenza com-
plications. Communication challenges in sustaining interest
and effort toward obtaining vaccination each year are
formidable, but may be more sustainable if vaccine supply
remains reliable in future years.
Trends in vaccination coverage among HCWs indicate
some progress in improving coverage in this group. Health
care workers can spread the highly contagious influenza
virus to patients and may have contributed to the death of
patients [37—39]. Despite being recommended annually for
health care workers by ACIP, and being part of the Standards
for Adult Immunization Practices , influenza vaccination
coverage among health care workers was not optimal.
Moreover, approximately 70% of health care workers stated
they had worked despite having influenza-like symptoms,
further highlighting the importance of influenza prevention
efforts, including following infection control measures
in this group . Although rates of health care worker
vaccination are low, with moderate efforts, organized
campaigns can obtain much higher rates of vaccination
among this population [25,42].
Pregnancy can increase the risk for serious medical
complications from influenza infection [43—46]. In 1997,
the ACIP recommended that women who will be in the
second or third trimester of pregnancy during the influenza
season need to get the influenza vaccination . In 2004,
the recommendation was expanded to include all women
who are pregnant during the influenza season . Pregnant
women were the least likely of recommended adult target
populations to be vaccinated, ranging from 9.3% to 14.4%,
1997—2004. In a study of influenza vaccination during
pregnancy, 22% had discussed influenza vaccine with their
physicians during pregnancy, with only 8% of respondents
having been vaccinated . Concerns about safety of
any drug or vaccine use during pregnancy likely affects
willingness of pregnant women to seek vaccination. How-
ever, no risk to pregnant women or their unborn children
from influenza vacation has been demonstrated [49,50].
In order to improve vaccine coverage, the Ob/Gyn role as
the primary care provider who vaccinates pregnant women
should be promoted and successful strategies need to be
identified in these settings.
Decreasing transmission of influenza from household con-
tacts to persons at high risk might reduce influenza-related
deaths among persons at high risk. Influenza transmis-
sion in households is a subject of renewed interest. One
study showed that overall in 279 households in which a
person was diagnosed with influenza, 131 (24.1%) sec-
ondary influenza cases occurred among the 543 households
contacts . ACIP recommends that close contacts of per-
sons at high risk for complications from influenza should
receive influenza vaccine . Limited information is avail-
able regarding use of influenza vaccine among household
contacts. Approximately 70 million persons aged 18—64
years without other indications for influenza vaccination are
household contacts in the U.S. . But this study showed
the coverage among household contacts aged 18—64 years
was only 21.1%. Providers need to recommend and pro-
vide vaccinations to household contacts. Further studies
are needed to determine why providers are not encour-
aging more strongly influenza vaccination for household
The findings in this study indicate a marked differ-
ence in influenza vaccination coverage by race/ethnicity.
Non-Hispanic whites were persistently more likely to be
vaccinated than non-Hispanic blacks and Hispanics among
persons aged ≥65 years, high-risk persons aged 18—64 years,
health care workers, and household contacts. For pregnant
women only non-Hispanic whites had a large enough sample
size for reliable estimates of coverage. Our results concur
with the findings of other studies that have documented
racial/ethnic differences in influenza vaccination in the
United States [21,24,25,53—55]. These differences may
result from a combination of factors, including differences
in attitudes toward vaccination and preventive care,
differences in patient—provider interactions, differences in
propensity to seek and accept vaccination, and differences
in quality of care [24,25,53—59]. Further studies are needed
to examine the contribution of other factors so that we can
more fully understand the complex causes of these patterns
and especially ways to overcome barriers blocking higher
of the results. All data for this study were collected by self-
report by an adult family member and influenza vaccination
status and high-risk conditions were not validated against
medical records. However, adult self-reported vaccination
status has been shown to have high sensitivity and moder-
ate specificity , with a range of 5—11% net over reporting
bias. A second limitation is that we cannot directly assess
season-specific estimates from 1989 to 2004 because individ-
uals were asked ‘‘During the past 12 months, have you had a
flu shot?’’ From 2005 on, ‘‘Which month and year did you get
your most recent flu shot’’ was added to the questionnaire
that will allow us to more accurately evaluate influenza vac-
cination coverage by season. In 2005, estimated coverage
for the 2004—2005 season among persons aged ≥65 years
based on reported month/year (September 2004—January
2005) was similar to coverage based on February—August
interview data (58% vs. 60%). A third potential limitation is
that we identified pregnant women based on being pregnant
at the time of the survey; information on the stage of preg-
nancy or due date was not available. In addition, we could
measure influenza vaccination coverage on only a subset of
household contacts of high-risk persons. Finally, informa-
tion was not available for some high-risk conditions (such
as neurological-related conditions that impair lung function)
identified by ACIP.
Substantial improvement in annual influenza vaccination
of currently recommended groups, their household contacts
and healthcare workers, and elimination of racial and ethnic
disparities in influenza vaccine coverage are needed to max-
imally reduce the health impact of influenza. Strategies to
improve coverage in adults include institution of reminder-
recall systems, use of media promotions and educational
programs, implementation of standing orders programs,
and utilization of alternative, convenient locations in
addition to medical settings for adults to obtain annual
influenza vaccine, such as worksite vaccination programs
1792 P. Lu et al.
Influenza vaccination coverage (%) for recommended adult populations, National Health Interview Survey (NHIS), 1989—2005a
NHIS survey yearb
65 or older
18—49 With high-risk
50—64 With high-risk
HH contacts (18—64)d
aThis table is based on February—August interviews only.
bEach year includes persons interviewed during February—August of the year and approximates coverage for the prior fall-winter vaccination period (2005 means 2004—2005 season,
and so on).
cData were not available.
dHousehold contacts are persons aged 18—64 years living in households with at least one identified person at high risk of complications from influenza infection including children <2
years. Vaccination of contacts of children <2 years was just encouraged for 2002—2003 and 2003—2004 seasons, and then fully recommended for the 2004—2005 season of vaccine shortage.
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