& Public Health
Copyright © 2013 The Korean Society for Preventive Medicine
J Prev Med Public Health 2013;46:62-73 • http://dx.doi.org/10.3961/jpmph.2013.46.2.62
Epidemiological Investigation of an Outbreak of Hepatitis
A at a Residential Facility for the Disabled, 2011
Hyun-Sul Lim, Kumbal Choi, Saerom Lee
Department of Preventive Medicine, Dongguk University College of Medicine, Gyeongju, Korea
Objectives: An outbreak of hepatitis A occurred at a residential facility for the disabled in July 10, 2011. This investigation was carried
out to develop a response plan, and to find the infection source of the disease.
Methods: A field epidemiologist investigated the symptoms, vaccination histories, living environments, and probable infection sourc-
es with 51 residents and 31 teachers and staff members. In July 25, 81 subjects were tested for the hepatitis A virus antibody, and
specimens of the initial 3 cases and the last case were genetically tested.
Results: Three cases occurred July 10 to 14, twelve cases August 3 to 9, and the last case on August 29. Among the teachers and staff,
no one was IgM positive (on July 25). The base sequences of the initial 3 and of the last case were identical. The vehicle of the outbreak
was believed to be a single person. The initial 3 patients were exposed at the same time and they might have disseminated the infec-
tion among the patients who developed symptoms in early August, and the last patient might have, in turn, been infected by the ear-
ly August cases.
Conclusions: The initial source of infection is not clear, but volunteers could freely come into contact with residents, and an infected
volunteer might have been the common infection source of the initial patients. Volunteers’ washing their hands only after their activi-
ty might be the cause of this outbreak. Although there may be other possible causes, it would be reasonable to ask volunteers to wash
their hands both before and after their activities.
Key words: Hepatitis A, Disease outbreaks, Hand disinfection, Voluntary workers, Residential facilities
Received: July 16, 2012 Accepted: December 26, 2012
Corresponding author: Hyun-Sul Lim, MD, PhD
123 Dongdae-ro, Gyeongju 780-714, Korea
Tel: +82-54-770-2401, Fax: +82-54-770-2438
This is an Open Access article distributed under the terms of the Creative Commons
Attribution Non-Commercial License (http://creativecommons.org/licenses/by-
nc/3.0/) which permits unrestricted non-commercial use, distribution, and repro-
duction in any medium, provided the original work is properly cited.
Hepatitis A is a viral infection transmitted via the fecal-oral
route. It does not show chronic progress, but rather has an
acute manifestation. After childhood infection with the hepa-
pISSN 1975-8375 eISSN 2233-4521
titis A virus (HAV), it progresses asymptomatically. After pro-
ducing the antibody for the HAV, symptoms do not recur even
after exposure to HAV. However, the progression of an HAV in-
fection in adulthood is serious. Thus, fatal cases could be oc-
curred without intensive care. Before the 1980s, most people
in their 20s had the antibody for hepatitis A because the water
and sewage system was incomplete and feces were used as
fertilizer in South Korea [1,2]. However, as the chance of expo-
sure to HAV decreased with expanding infrastructure and im-
proved hygiene, the number of adults who had not been in-
fected with HAV in childhood increased, and the seropositive
rate for the HAV antibody in people in their 20s continuously
decreased [3,4]. As a result, in the late 1990s, the HAV infection
spread rapidly among the adolescent and young adult popu-
Hepatitis A Outbreak Investigation
lation [1,3,5]. Under these conditions, a strategy for dealing
with HAV infection is necessary because additional outbreaks
of hepatitis A can continue to be expected.
The occurrence of HAV infection in three disabled residents
of a residential facility was reported to a public health center
from a hospital located in P city on July 20, 2011. Therefore, an
epidemiological investigation was conducted to identify the
infection source of the HAV and exact transmission route and
to prevent the further spread of the virus.
Facility A, in which the hepatitis A occurred, is for the severely
disabled. Residents in this facility spend most of their time in-
side the facility. Some of them do outside activities such as go-
ing to a school for the disabled and going to church. Consider-
ing the conditions of the group residential facility, a complete
enumeration investigation was performed with a total of 82
people (51 residents, 13 teachers, and 18 staff members). Al-
though visitors and volunteers who came into contact with
the residents are supposed to be included in this investigation,
a telephone interview was performed with visitors to examine
their symptoms and diagnoses. Considering the incubation
period for HAV, volunteers were excluded from the present
study because the range of subjects would have extended to
include about 200 volunteers a month. Instead, the visiting
and volunteering histories were gathered from the hepatitis A
patients who had been reported to the K and P cities.
The epidemiological investigation team was organized from
the public health center in K city, which includes facility A. The
primary investigation was performed by a field epidemiologist
of the public health center on July 20. For most of the residents,
bathroom assistance from the staff and volunteers and wear-
ing a diaper were needed due to physical or mental impair-
ment. For this reason, additional cases were likely. The second-
ary investigation was conducted by the Korea Centers for Dis-
ease Control and Prevention (KCDC) epidemiological investi-
gation department, the epidemiological investigation team in
the public health policy department of the jurisdictional prov-
ince government, and the epidemiological investigation team
of the public health center in K city on July 25 (Figure 1).
Case monitoring and transmission control
The three cases reported on July 20 had been hospitalized
on July 16. Since the primary investigation, active surveillance
system had been activated. The anti-HAV immunoglobulin
test was conducted on July 25, which was the secondary in-
vestigation date. The results of the antibody test were report-
ed on July 29; however, a sufficient supply of vaccine for 52
people was difficult to obtain at once. The disabled residents
were vaccinated first since they were a vulnerable group due
to their personal hygiene, while teachers were able to manage
their personal hygiene with methods such as hand washing.
At first, 36 residents seronegative for IgG anti-HAV were vacci-
nated on July 29, which was the date when the antibody test
report was made. Next, 16 staff members and teachers with-
out the antibody were vaccinated on August 3. After finishing
the vaccination to group without the antibody on August 3,
the active surveillance was converted to passive surveillance.
The day when the first resident was discharged from the hos-
pital was July 27. After July 27, residents who were discharged
from the hospital and those with symptoms were isolated for
monitoring. Isolated residents used a separate restroom in the
isolation room. Teachers seropositive for IgG anti-HAV were as-
signed to these residents. Considering the period needed for
antibody production , they were isolated until September 3,
which was 4 weeks after August 3, which, in turn, was the last
vaccination date in the facility.
Interview and living environmental investigation
In the individual interviews, the following information was
Figure 1. Timeline of epidemiological investigation. The pa-
tient with the index case was diagnosed in July 20. 1A serio-
logic test and genomic test were done. 2Case investigation,
questionnaire, and education were performed.
Vaccination (5 staff
members and 11
(Aug 3 to Aug 9)
12 Case developed
Case reports & primary
End of the
Hyun-Sul Lim, et al.
collected: the hepatitis A vaccination history, clinical symp-
toms, history of contact with hepatitis A suspected patients,
history of travel abroad, conditions of water distribution in the
facility, dietary history, resident’s outdoor activities, and histo-
ry of eating and drinking outside the facility. However, the
conversations with residents were difficult because most of
them were mentally impaired, and thus could not answer most
of the interview questions. Therefore, most of the interview
questions were answered by the teachers. Only clinical symp-
toms could be investigated in the individual interviews with
residents. Information collected from the teachers for the anal-
ysis included the history of traveling abroad, water distribution
in the facility, eating history, and outdoor activities for the dis-
abled residents. However, collecting information on the resi-
dents’ outside eating and drinking history and history of con-
tact with people suspected of having hepatitis A was impossi-
ble. Detailed information such as the floor plan of the living
unit, daily schedules, and the pattern and frequency of contact
among subjects was examined.
Investigation of clinical and environmental specimens
On July 25, for clinical specimens, blood sampling was con-
ducted from 51 disabled residents, 17 teachers (one of the 18
teachers, who had an HAV vaccination history, was excluded),
and 13 staff members. The stool specimens were collected
from the three confirmed patients to be sent to the KCDC for
tests. For the blood tests, antibody tests for HAV IgM and IgG
in 81 specimens and HAV genetic tests were performed in the
three confirmed cases. The hepatitis B virus serologic test was
also performed in one patient with hepatitis B. The HAV genet-
ic test was conducted from stool specimens. For the genetic
test of the blood and stool, a reverse transcription polymerase
chain reaction (RT-PCR) test was performed. This test was con-
ducted using eight Korean strains of HAV. Three of the eight
strains (2010-HAV-JH-VP3-, 2010-HAV-JH-VP3-, and 2010-
HAV-JH-VP3-) were acquired from patients in Jeju island,
three more (EU049550, EU049563, and EU049548) were from
Gyeonggi province, one of them (KNIH-GS-07-01) was from
Gaeseong industrial district, and one of them (KNIH-JN-08-01)
was from South Jeolla province. During the monitoring period,
additional patients were immediately hospitalized and given
the HAV IgM antibody test; however, the genetic test was not
performed. The last case occurred on August 29, and genetic
tests from the blood and stool were performed for this case.
Microbiological tests were performed for Salmonella species,
Shigella species, Vibrio species, Escherichia coli, Staphylococcus
aureus, Listeria monocytogenes, Campylobacter jejuni, Clostridi-
um perfrigens, Bacillus cereus, and Yersinia enterocolitica with
cooking utensils including kitchen knives, cutting boards, and
dishcloths as environmental specimens. Filtered water as a
source of drinking water, cooking water, and ground water was
sent to the Institute of Health Environment and the public
health center in the autonomous district to test for potability.
Color, turbidity, odor, taste, NHn-N, NO3-N, total colony counts,
total coliforms, fecal coliforms, KMnO4 consumption, total sol-
ids, pH, Cl-, Al, Mn, and F were evaluated in the test. Testing of
preserved foods was not possible to perform because it was
difficult to estimate the date of exposure of these foods, and
the HAV incubation period was long.
A hepatitis A case was primarily defined as case that occurred
among the disabled residents, teachers, staff members, and
visitors in facility A since July 10, 2011, which was the date of
the attack in the initial case. The final case definition was de-
termined again with the results of the serologic test. Consider-
ing the incubation period, a hepatitis A case was defined as
occurring among the disabled residents, teachers, and staff
members of facility A after June, 2011, with a combination of
IgM anti-HAV seropositivity from the antibody test and at least
one of the following symptoms: fever, chills, myalgia, nausea,
vomiting, diarrhea, abdominal pain, fatigue, loss of appetite,
itching, dark urination, and jaundice. The date of the occur-
rence of the hepatitis A was defined as the date of the onset of
Subclinical infection investigation
A subclinical infection was defined as a case among the dis-
abled residents, teachers, and staff members of facility A since
June, 2011 with IgM anti-HAV seropositivity in the antibody
test but without any of the following symptoms: fever, chilling,
myalgia, nausea, vomiting, diarrhea, abdominal pain, fatigue,
loss of appetite, itching, dark urination, and jaundice.
The attack rate by sex was compared using Fisher’s exact
test. The attack rates by age, type of subject (disabled resident,
teacher, or staff member), and living unit was analyzed with
the log likelihood test because the expected frequencies of
the attack rates were insufficient to use the chi-squared test.
Hepatitis A Outbreak Investigation
All statistical analyses were performed using SPSS version 17.0
(SPSS Inc., Chicago, IL, USA). To calculate the 95% confidence
interval of the attack rate with the following equation, the val-
ues of the quadratic equation were calculated by assigning π,
which was the population fraction, to an unknown variable.
These values were for the minimum and the maximum values
of the 95% confidence interval . The equation used is as fol-
(χ2=3.84, p=sample fraction, n=sample size)
The initial case occurred on July 10, and two additional cas-
es occurred on July 14. During the active and passive surveil-
lance, three cases occurred on August 3, another three cases
on August 4, a case on August 5, three cases on August 6, and
two cases on August 9. With the last case on August 29, the
total number of cases came to 16 (Figure 2).
Result of Environmental Investigation and
Attack Rate by Resident’s Living Unit
The 51 disabled residents lived in 7 living units. A recreation
hall was located in the center of the building. There were four
living units named A, B, C, and D and three living units named
E, F, and G in the hallways on both sides (Figure 3). Each unit
(units A and B, units C and D, and units F and G) was connect-
ed with another through balconies; however, the pairs of units
A and B and units F and G each included one unit with report-
ed cases and one with no case (Figure 3). Each unit had two
rooms and a bathroom; each room had a door connecting to
the bathroom that had two toilets and a shower.
The male and female residents did not enter units for the
opposite sex; however, they could enter other units for the
same sex. In addition, they freely moved around the recreation
hall and hallways. In addition, they were able to come in con-
tact with each other easily during programmed events and
regular daytime activities. There were various programs, and
most of them were group programs performed in an assembly
hall and an audiovisual room.
By unit, there were 6 cases out of 7 susceptible persons
(85.7%) in unit C, 5 out of 8 (62.5%) in unit B, two out of 6
(33.3%) in unit E, and one out of 4 (25.0%) in each of units D
and G, and the differences among units were statistically sig-
Figure 2. Cases of hepatitis A at living facilities for the disabled, by time of onset, July to August 2011. Each square denotes a pa-
tient and his/her living unit number in the corresponding square on the curve. The base sequences of 4 cases in July 10, July 14,
and August 29 were identical. The other cases were not tested by reverse transcription polymerase chain reaction.
Rec eation Recreation
Unit GUnit G
● Cases in Jul 10 and Jul 14
● Cases from Aug 3 to Aug 9
● Last case (Aug 29)● Last case (Aug 29)
◎ Susceptible person not attacked
○ Immunized person
Figure 3. Floor plan of accommodations. Males and females
lived in separate living unit; units A to E for male, units F and
G for female. Units A and B, units C and D, and units F and G
are connected by balconies.
The case of the male teacher
Hyun-Sul Lim, et al.
nificant (p=0.003). No cases occurred in unit A or F (Table 1,
All of the 16 cases complained of fever. In order of frequen-
cy, there were 8 cases (50.0%) with fatigue; 7 cases (43.8%)
each of loss of appetite, nausea, and vomiting; 6 cases (37.5%)
of the chills; abdominal pain in 5 cases (31.3%); and jaundice
in 3 cases (18.8%). Three cases with jaundice occurred on July
10 and 14 and August 3 (data not shown).
Results of Drinking and Cooking Water and
Institutional foodservice was available for the disabled resi-
dents, teachers, and staff members. The same menus were
provided to all of them during the two month period. Food
trays, spoons, and chopsticks were used in common without
separation among the groups. Due to the lack of a water sup-
ply system, filtered water from groundwater and boiled water
were used for drinking water. Groundwater was used for cook-
ing. The disabled residents were found to drink groundwater
from the bathroom.
According to the results of the investigation of drinking and
cooking water and cooking utensils in Table 2, bacteria were
not found from the bacteria test on kitchen knives, cutting
boards, or dishcloths. Nor were bacteria found in the filtered
water and groundwater for drinking and cooking; however,
the color and the turbidity of the cooking water and ground-
water were judged to be unfit for drinking. The report on the
47-item drinking water quality examination of groundwater
from the P city waterworks had determined the groundwater
to be suitable for drinking on December 22, 2010; however, it
was then found to be unsuitable due to color, turbidity, iron,
and manganese on July 5, 2011. This result was corresponded
with the color and turbidity results of the present study.
The kitchen was examined by the sanitary instructor team
of the public health center, and none of the foodservice em-
ployees were found to have injuries on their hands or com-
plaints such as diarrhea. Health examinations were performed
on all of the foodservice employees. There was a possibility
that visitors could bring food in from outside; however, there
was no chance that the three cases of July could have eaten
Result of Clinical Specimen Investigation
From the antibody test on July 25, 15 residents out of 51
were identified to be seropositive for IgG anti-HAV. The three
confirmed cases with hepatitis A showed IgG. Since they
showed symptoms beginning on July 10 to 14, it can be con-
cluded that the IgG antibody was converted to positive around
July 25, which was the date of the blood sampling. The infec-
tion risk in the other IgG positive 12 residents was considered
to be low when these residents were exposed to HAV. Besides
the three confirmed cases, an additional subject showed sero-
positivity in the IgM anti-HAV test. However, this subject had
no symptoms, and the result of the genetic test was negative.
Among the 18 teachers and 13 staff members, 30 subjects, that
is, all except one person with a vaccination history, underwent
the serologic test. Fourteen cases of IgG anti-HAV seropositivi-
ty were confirmed, and there was no subject with IgM anti-HAV
positivity (Table 1).
To confirm hepatitis A, an additional antibody test was per-
formed in the cases that occurred after the antibody test con-
ducted on July 25. Thirteen cases with symptoms were con-
firmed with IgM anti-HAV seropositivity from the additional
test. The results of the serologic test relative to the termination
point of the epidemic are shown in Table 1.
HAV RNA was detected in the initial three cases, and their
genotype was confirmed with IA. These three cases were in-
fected from the same source because the genomic sequence
similarity corresponded 100% in each of their three strains
(2011-KJO-WTK-2-14, 2011-KJO-JHS-2-23, and 2011-KJO-OHJ-
2-30). The percent identity in common with existing Korean
strains was 94.1% to 96.8%. HAV RNA were found in two out
of three stool specimens.
The genomic sequence similarity in the additional strain
from the case that occurred on August 29 corresponded 100%
with the strains from the initial three cases from July 10 to 14.
By this result, it was confirmed that this case (which occurred
on August 29) was disseminated from the same virus. HAV RNA
was also found in the stool specimen of this additional case.
General Characteristics and Attack Rate
The subjects’ general characteristics and attack rates by age,
sex, and work position are shown in Table 1. Eleven of the resi-
dents, 6 teachers, and 8 staff members out of 81 showed IgM
seronegativity and IgG seropositivity. One teacher with an
HAV vaccination history was not tested; however, this individ-
ual was considered to have immunity. Thus, 26 people were
Hepatitis A Outbreak Investigation
Table 1. The results of serology test and hepatitis A attack rate by subject (residents, teachers/staff, and total)
Initial test on 25 July (n=81)
Until the end of
51 11 40 1537.5 (24.2, 53.0)
45.2 (29.2, 62.2)
11.1 (2.0, 43.5)
58.3 (32.0, 80.7)
22.2 (9.0, 45.2)
50.0 (21.5, 78.5)
0.0 (0.0, 65.8)
0.0* (0.0, 43.4)
62.5 (30.6, 86.3)
85.7 (48.7, 97.4)
25.0 (4.6, 69.9)
33.3 (9.7, 70.0)
0.0 (0.0, 39.0)
25.0 (4.6, 69.9)
6.3 (1.1, 28.3) 311516
11.1 (2.0, 43.5)
0 (0.0, 35.4)11
9.1 (1.6, 37.7)
0 (0.0, 49.0)
0 (0.0, 79.3)
28.6 (18.4, 41.5) 2616
37.5† (24.2, 53.0)
6.3 (1.1, 28.3)
58.3* (32.0, 80.7)
17.2 (7.6, 34.5)
33.3 (13.8, 60.9)
0.0 (0.0, 56.1)
37.5* (24.2, 53.0)
9.1 (1.6, 37.7)
0.0 (0.0, 43.4)
HAV, hepatitis A virus.
1Sum of all the IgM anti-HAV test results received over the surveillance period.
2People who were IgM anti-HAV negative and IgG positive in the serology test on July 25. One person (teacher) who was vaccinated before was included.
3Initial 3 cases and 1 asymptomatic infection were regarded as the susceptible persons although they were IgG anti-HAV positive.
495% confidence intervals of attack rate are the value (π) of the following quadratic equation ‘χ2=(p-π)2/[π(1-π)/n] (χ2=3.84, p=proportion of sample, n=number
5Males’ living unit.
6Females’ living unit.
*p<0.05 calculated by log likelihood test, †p<0.05 calculated by Fisher’s exact test.
Hyun-Sul Lim, et al.
excluded from being counted in the attack rates (Table 1).
According to the case definition, the number of cases was
16 (15 disabled residents and one teacher) among all of the
subjects, and the number of susceptible persons was 56.
Therefore, the attack rate was 28.6%. There were 15 cases out
of the 40 males (37.5%) and one case out of the 16 females
(6.3%). The attack rate in the males was significantly higher
than in the females (p=0.023). There were 15 cases out of the
40 residents (37.5%), one case out of the 11 teachers (9.1%),
and no one among the staff. The attack rate in the disabled
residents was high and statistically significant (p=0.025). By
the age, the attack rate in the teens was 7 cases out of 12
(58.3%) and that of those in their 30s was 4 cases out of 12
(33.3%); thus, their attack rate was high (p=0.033) (Table 1).
The attack rate in the disabled residents was 37.5%. There
were 14 cases out of the 31 males (45.2%) and one case out of
the 9 females (11.1%). Although the attack rate in the males
was higher than in the females, the difference was not signifi-
cant (p=0.117). By age, the attack rate in the teens was 7 cases
out of 12 (58.3%) and those in their 30s was 4 cases out of 8
(50.0%). Their attack rates were high; however, there was no
significant difference (p=0.091) (Table 1).
Subclinical Infection Rate
Besides the 16 cases confirmed by serology and symptoms,
one subject was positive for IgM anti-HAV. However, this sub-
ject was classified as having a subclinical infection because no
symptoms occurred. The number of confirmed cases was 16,
and the subclinical infection rate was 5.9% (1/17).
Outside Activity of Cases Occurring in July
The possibility of being exposed to HAV from the outside
was examined in the three cases occurring from July 10 to 14.
Among the outside activities of disabled residents in facility A,
some residents went to religious services and attended one of
the two special schools. Two male cases out of the three that
occurred in July shared a unit, and they rarely participated in
outside activities. On the other hand, the disability of the fe-
male resident was not severe; thus, she was active outside the
facility. She participated in religious activities and also attend-
ed a special school. There was no opportunity in which all
three of these cases could participate in outside activities si-
multaneously during the estimated risk period. Furthermore,
there had been no report of hepatitis A in the places where
the resident participated in outside activities. The occurrence
of hepatitis A was examined by visiting the two special schools
that the residents of facility A attended on October 4, and there
was no occurrence of hepatitis A found.
History of Contact with Insiders and with
Two teachers were assigned in a unit to assist disabled resi-
dents. An additional four teachers moved among the units as
alternate care givers. The male teachers were permitted to en-
ter males’ units, and the female teachers were permitted to
enter the units of both sexes. After confirming the outbreak of
hepatitis A, teachers positive for IgG anti-HAV assisted the in-
fected cases and supervised residents to minimize the contact
with one another during the outbreak period. The only male
teacher who contracted hepatitis A had worked in units A, B, C,
and E from July 1 to 16 and in units A, B, C, and D from July 17
to the date of symptom onset.
Visitors who were relatives of residents had no opportunity
to come in contact with all of the three cases that occurred in
the middle of the July at the same time. No visitor had been
diagnosed with hepatitis A or its symptoms during the expect-
ed exposure period (May 20 to June 25) for the cases that oc-
curred in July, according to the results of telephone interviews.
Volunteers mainly assisted with and bathing the residents
and cleaning. They cleaned outside of the living units. Volun-
teers assisted the disabled residents of the same sex in bath-
ing in the bathroom located in the unit. Contact with the op-
posite sex was not limited among the volunteers and residents
except at bath time; thus, the volunteers were available to
come into contact with residents of the opposite sex in places
Table 2. The results of environmental sample examination
Items exceeding the standard
1Tests included Salmonella species, Shigella species, Vibrio species, Esch-
erichia coli, Staphylococcus aureus, Listeria monocytogenes, Campylobacter
jejuni, Clostridium perfrigens, Bacillus cereus, and Yersinia enterocolitica.
2Tested items were color, turbidity, odor, taste, NHn-N, NO3-N, total colony
counts, total coliforms, fecal coliforms, KMnO4 consumption, total solids, pH,
Cl-, Al, Mn, and F.
Hepatitis A Outbreak Investigation
other than the residents’ units. The volunteers came from two
areas, K and P cities, and the number of residents of P city was
higher than that of K city. Those with a volunteer history at fa-
cility A were searched for among the cases of hepatitis A re-
ported to K and P cities; however, no volunteers appeared in
the list of reported cases.
It was concluded that the causal pathogen of this outbreak
was HAV because 16 cases with clinical symptoms of hepatitis
A showed IgM anti-HAV seropositivity in a serologic test. HAV
strains can have one of seven genotypes. Four of them (I, II, III,
and VII) have been detected in the human body ; the other
three of them (IV, V, and VI) have been isolated from animals
. The genotype isolated from this outbreak was IA, which
was detected from cases in the early 2000s [10,11].
RT-PCR tests were performed in the initial three cases that
occurred from July 10 to 14 and the last case that occurred on
August 29. Their genotypes and genomic sequences from the
strains were 100% identical, which means they were dissemi-
nated from the same virus.
The original source of infection at facility A could be either
of two possibilities. One option would be from foodborne or
waterborne pathogens. Another source could be from the
contact with a person having an infection history.
Waterborne infection was possible because groundwater,
filtered water, and boiled water were used for drinking, and
groundwater was used for cooking with no water filtration
system at facility A. However, the possibility of waterborne in-
fection was less likely because cases of waterborne infection
would continuously occur during the same period. The possi-
bility of infection by cooking water and cooking utensils was
also not very high. The same menus were provided by institu-
tional foodservice to the disabled residents, teachers, and staff
during the two-month period. Food trays, spoons, and chop-
sticks were used for common. Because 82 people used the
same food trays, spoons, and chopsticks, and only three cases
occurred three weeks earlier than the other cases, infection by
cooking utensils was unlikely. The possibility of infection by
food from outside was also low because there was no way that
the initial three cases could have eaten outside food together.
If cases had been disseminated from filtered drinking water,
cooking water, or food, the size of the outbreak would have
been greater, and a unimodal distribution on an epidemic
curve would be shown . Therefore, the possibility of infec-
tion by water and food can be excluded.
The other possibility is from person-to-person contact. The
interval of occurrence of the cases was about three weeks: July
10 to 14, August 3 to 9, and August 29. In addition, a long and
irregular pattern was shown on the epidemic curve. Therefore,
infection by a personal contact was suspected in this outbreak.
Teachers, staff members, visitors, and volunteers could be con-
sidered potential transmission vectors inside the facility. Cases
could be disseminated from teachers; however, there is no
possibility that this occurred because no teacher had symp-
toms before the occurrence of the initial three cases, and there
was no teacher with subclinical infection according to the re-
sult of the serologic test on July 25. The same logic could be
applied to the staff. The possibility of infection from visitors is
less likely because there was no opportunity in which the ini-
tial three cases that occurred in July could have simultaneous-
ly come into contact with any of the visitors.
Contact with the opposite sex was not limited for volunteers
except at bath time. Furthermore, volunteers were available to
come into contact with residents outside the living units with-
out any limitations. Thus, if there were patients in the incuba-
tion period or with subclinical infection among the volunteers,
there could have been a possibility that the initial three cases
were simultaneously disseminated from them. However, there
was no one who responded as being involved as a volunteer
at facility A among the cases of hepatitis A reported to K and P
cities. Therefore, it cannot be concluded with certainty that a
volunteer was the primary infection source. In sum, the possi-
bility that the HAV outbreak at facility A could have been start-
ed by person-to-person spread is high, and volunteers are sus-
pected to be the most likely infection source.
The interval of the three cases that occurred from July 10 to
14 was only four days; thus it is difficult to conclude that any
of these cases could have been disseminated from one anoth-
er. Furthermore, the case that occurred on July 10 had jaun-
dice on July 20, and the case that occurred on July 14 had
jaundice on July 21. The infectious period is around 2 to 3
weeks before jaundice to a week after jaundice, or two weeks
before symptoms to a week after symptoms [13,14]. Therefore,
the possible infectious period of one case from another would
be from July 1 to 16; July 16 was the date of hospitalization of
the three cases. The mean incubation period of hepatitis A is
28 days, and the possible incubation period is from 15 days to
50 days . As a result, it can be assumed that these three
Hyun-Sul Lim, et al.
cases were exposed during the same period. Calculating the
exposure date by the mean incubation period, the exposure
date should have been between June 12 and 15. Using July 10
as the reference, the possible exposure date would have been
between May 20 and June 25. Because the two male cases
shared unit B, they could have been infected at the same time
by a person-to-person route. While a female case lived in unit
G, her case could have been disseminated by the same person
due to her active outside activity. Thus, it is assumed that the
initial three cases were disseminated by personal contact from
the same person.
Twelve cases occurred between August 3 and 9 and showed
IgM anti-HAV seronegativity in the antibody test on July 25.
This antibody is found 5 to 10 days after exposure to the anti-
gen . Considering the estimated exposure period, these 12
cases could not have been exposed during the same period as
the initial three cases. Considering the infectious period of the
cases that occurred in July (from July 1 to 16), and the incuba-
tion period, it is reasonable to assume that the 12 cases were
infected by the initial three cases. The male teacher who took
care of the initial three cases after hospitalization showed IgM
anti-HAV seronegativity in the antibody test on July 25; how-
ever, after symptoms appeared, the result was converted to
IgM positivity on August 6. It is assumed that the male teach-
er’s case was disseminated from the initial three cases. Howev-
er, the cases that occurred between August 3 and 9 could have
been infected by mutual spread. One case that occurred on
August 3 was hospitalized with jaundice on August 5 (Hospi-
talization was unavailable on August 3 due to a shortage of
patient rooms. Thus, this case was isolated in facility A); thus,
the possible infectious period of this case was between July 15
and August 3.
The last case that occurred on August 29 had been vaccinat-
ed on July 29 because IgM and IgG were negative in the sero-
logic test on July 25. The preventive effect of the hepatitis vac-
cine is higher than 94% a month after the initial vaccination
. Thus, it was concluded that this case was disseminated
from the residents or the male teacher with an infectious peri-
od in August after the vaccination but before formation of IgG
anti-HAV. In this last case, a vaccination failure issue should
also be considered. Vaccination failure means that a target
pathogen (antigen) develops a disease in spite of being vacci-
nated, and this condition is caused by one of the following
two reasons: vaccine failure and failure to vaccination. Vaccine
failure means that the disease develops despite taking into ac-
count the incubation period and the normal delay for the pro-
tection to be acquired. Other cases of vaccine failure are those
in which the vaccination effect is not certain for the specific
disease, or the particular marker of protection is not produced
. Failure to vaccinate is caused by administrative error, stor-
age failure, or an inappropriate time and number of primary
and/or booster vaccinations . Thus, it could be concluded
that this case was caused by the failure to vaccinate rather
than vaccine failure because the interval between the expo-
sure date (August 1), which was estimated from the mean in-
cubation period, of the last case and the vaccination date was
not significantly different.
Because no additional case had occurred after the last oc-
currence date plus the longest possible incubation period, and
the last vaccination was performed on August 3, the end of
the outbreak was declared on October 21, 2011.
To calculate the attack rate, the number of cases is divided
by the number of exposed persons; however, in this study, the
number of cases was divided by the number of susceptible ex-
posed persons (a susceptible exposed attack rate was calculat-
ed). The measurement method is very important because it
affects the attack rate. The method used in the present study
is an objective method for measuring infectivity. However, this
method is difficult to apply without identified immunologic
results (antibody tests) .
Reviewing the attack rate by sex, the attack rate in male resi-
dents was significantly higher, and there was only a case
among the female residents because the female residents had
a smaller chance of coming into contact with the males, who
were using different units. Another reason is that the female
residents had fewer opportunities to come into contact with
the male teacher with symptoms.
For the attack rate by living unit, the attack rates of units B
and C were higher than those of the other units. The number
of people with immunity in units B, C, and E was one each, and
there was no one with immunity in unit F. Thus, the number of
susceptible people was relatively high, which means the attack
risk was high. Unit F was occupied by female residents, and
unit E, which was occupied by male residents, was distant from
the other males’ units. For these reasons, it is assumed that the
unit F and E attack rates were low. No cases occurred in unit A,
which was connected to unit B through a balcony; however,
an additional case was reported on August 29 in unit D which
was connected with unit C. The risk of spread through the bal-
cony would be low because moving through balconies was
Hepatitis A Outbreak Investigation
controlled after the occurrence of the initial case. Frequent
contact among the residents in units B and C could be a po-
tential reason; however, no concrete connection was found. It
could be assumed that no occurrence in unit A was caused by
the relatively low number of susceptible persons and less
chance of contact.
Estimating the transmission route among the units by the
male teacher’s working history, the male teacher had worked
in unit B in the early part of July, which was the period of the
expected exposure of the male teacher. He had worked in
units A, B, C, and D, and was mostly assigned to units B and D
beginning in the middle of July, which was the communicable
period. Explaining the transmission to units with only the male
teacher’s working history has limitations; however, it could be
assumed that he carried the virus to units B and C, places with
a relatively high number of susceptible people among units A,
B, C, and D. He could have spread HAV to the case in unit D,
where he was assigned relatively often.
The subclinical infection rate of 5.9% was lower than that
found in previous studies [17,18]. The reason for the lower rate
must be that an antibody test was performed in only new cas-
es with symptoms during the monitoring period. For a more
complete investigation, the antibody test should have been
repeated at the ending point of the investigation for subjects
with IgG and IgM seronegativity in the initial antibody test.
The attack rate could also be underestimated by ignoring mild
symptoms because asymptomatic infection was not consid-
ered. Furthermore, these asymptomatic healthy carriers may
have been the source of infection because they were not iso-
It could be estimated that this outbreak occurred due to
person-to-person spread; however, the initial source of infec-
tion was not identified. The only method for doing so would
have been to collect data from all the volunteers, who were
considered to be an infection source, based on the history tak-
ing on volunteer activity at facility A in the reported hepatitis
A cases. Another limitation of this study was that we did not
examine personal contact between local persons reported to
be infected and facility volunteers.
The reason volunteers were suspected to be the source of
infection was that most of them wash their hands only after
volunteering. During activities involving frequent contact with
people such as medical practice, the people performing these
activities do not always wash their hands before contact. The
cases of the volunteers are similar [19,20]. Although the same
behavior pattern could be found among the teachers, the pos-
sibility that the male teacher was the primary source of infec-
tion is extremely unlikely given the result of his antibody test.
To verify infection from the volunteers, analysis of detailed
volunteering schedules, records of volunteering activities, their
contact information, and an institutional strategy and social
atmosphere necessary to support contacting them for addi-
tional information would be necessary. However, it would be
difficult in practice to collect the volunteers’ personal contact
information and to contact them to examine whether symp-
toms exist or not. For these reasons, it could not be concluded
with certainty that volunteers who were in the incubation pe-
riod or had a subclinical infection spread the virus. Neverthe-
less, given its likelihood, sanitation for volunteers should be
emphasized. In particular, hand washing must be performed
before activities. It also should be repeated when they are
contaminated from urine, feces, and nasal discharge during
contact activities. Although it would be difficult to ask volun-
teers who visit to provide assistance for social reasons to per-
form hand washing, it is essential to prevent them from being
carriers for infectious diseases. Furthermore, it is necessary to
prevent themselves from contracting diseases.
The teacher who was the only male case besides the dis-
abled residents could have been infected and have been a car-
rier after caring for the initial three cases before they had been
diagnosed with hepatitis A. In addition, he could have spread
the virus by active contact with other residents after dissemi-
nation but before showing symptoms. Therefore, it is better
that a person with objectively confirmed immunity cares for
patients during an outbreak.
In this outbreak, it was possible to prevent the spread of the
disease by fast vaccination treatment after the first cases were
reported. Even though the teachers and staff were vaccinated
five days later than the residents because of an inadequate
vaccine supply, severe spreading could be prevented by vacci-
nating the vulnerable group first. To prevent hepatitis A, a
hepatitis A vaccine or immunoglobulin is injected. After com-
mon exposure, the hepatitis A vaccine is recommended for
healthy people aged from 12 months to 40 years, and the im-
munoglobulin inoculation is recommended for people aged
41 and over . Both the hepatitis A vaccine and immuno-
globulin are available to use in people who have intimate per-
sonal contact such as sexual contact and family relationships.
Susceptible persons would be exposed to the patient’s fe-
ces, in which the most pathogens would be present because
Hyun-Sul Lim, et al.
there were many people using diapers in facility A. Moreover,
the antibody could not be produced in the appropriate time
by vaccination alone because the initial cases occurred be-
tween July 10 and 14, and the date of vaccination was on July
29 and after, which was two weeks after the initial outbreak
. Therefore, simultaneous vaccination with the hepatitis A
vaccine and immunoglobulin could be considered for people
who changed a patient’s diaper or shared units with a hepati-
tis patient. The same would apply to similar situations.
To increase the vaccination rate for hepatitis A in adolescent
and adult groups involved in institutional foodservice and ac-
tive outdoor activities, policies for governmental support are
essential. To increase vaccine coverage rates in youth and
adult groups, additional campaigns and efforts targeting them
are necessary because the progress of hepatitis A is more seri-
ous in young adults and middle-aged people than in young
children, and there are fewer opportunities to be vaccinated in
youth and adult groups than in infants.
The primary limitation in this epidemiological investigation
is that most of the responses were acquired from teachers in-
stead of the residents themselves because face-to-face inter-
views with them were impossible due to the mental impair-
ment of most of the residents.
The next limitation is that accurate data collection by mem-
ory was difficult during the estimated exposure period because
the incubation period for hepatitis A is long.
Thirdly, a complete investigation was practically impossible
due to the large number of volunteers without personal con-
tact information and their irregular visiting schedules. Volun-
teer histories were investigated and compared to the reported
cases of hepatitis A of P and K cities; however, there was no re-
lationship. In addition, reported data may not be accurate be-
cause not every case of hepatitis A is reported.
Finally, the antibody test for hepatitis A was only performed
without a genetic test in the case that occurred in August and
in the remaining subjects without symptoms. Therefore, the
number of cases may have been underestimated. This limita-
tion could affect the subclinical infection rate. Furthermore, it
would be difficult to examine their exact relationships in all
Nevertheless, the present epidemiological investigation is
meaningful as a reference for future research because serolog-
ic data was collected from an almost complete enumeration in
a closed group. Moreover, a susceptible exposure attack rate
was calculated as a practical indicator of the attack rate, and
the subclinical infection rate was analyzed. In the attempt to
identify the source of infection, the possibility of infection by a
volunteer was assumed, and the importance of hand washing
before activities could be emphasized. Therefore, the possible
spread of diseases should be prevented by complete sanita-
tion when volunteering.
Special thanks is extended to the KCDC staff and all the peo-
ple who supported this epidemiological investigation.
CONFLICT OF INTEREST
The authors have no conflicts of interest with the material
presented in this paper.
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