Mumps outbreak in Israel’s highly vaccinated society:
are two doses enough?
E. ANIS1,2*, I. GROTTO1,3, L. MOERMAN1, B. WARSHAVSKY1, P. E. SLATER1,2
AND B. LEV1
1Ministry of Health, Jerusalem, Israel
2Braun School of Public Health, Hebrew University and Hadassah, Jerusalem, Israel
3Epidemiology Department, Faculty of Health Sciences, Ben-Gurion University of the Negev, Beer-Sheva, Israel
(Accepted 16 March 2011)
Mumps outbreaks in recent years have given rise to questions about the effectiveness of the
mumps vaccine. This study examined the epidemiological data from a recent mumps outbreak in
Israel and from outbreaks in other countries with high vaccination coverage, and considered
whether long-established vaccination policies designed to protect against mumps are in need of
revision. Of over 5000 case patients in the Israeli outbreak, half of whom were in the Jerusalem
health district, nearly 40% were aged o15 years and, of those whose vaccination status was
known, 78% had been fully vaccinated for their age – features similar to those in recent mumps
outbreaks in Europe and North America. The epidemiological and laboratory evidence suggests
that many previously vaccinated adolescents and young adults are now susceptible to mumps
because their vaccine-based immunity has waned. Booster vaccination programmes for those at
high risk of infection during mumps outbreaks – particularly those in congregate living
environments – merit priority consideration.
Key words: Age cohort, mumps outbreak, vaccination, waning immunity.
Dramatic reductions in the incidence of mumps have
occurred wherever the mumps vaccine was made a
routine component of paediatric healthcare. In the
WHO European region declines in incidence have
ranged from 88% to 99% . Nevertheless, after a
series of recent mumps outbreaks that involved
large numbers of previously vaccinated individuals,
existing mumps vaccination programmes in Western
Israel’s experience with mumps has paralleled that
of other highly vaccinated societies. In the pre-
vaccination era mumps incidence had averaged over
4500 cases annually and ranged from 20 to 162 cases/
100000 population. In 1989 routine immunization
with theMMRvaccine was introduced atthe ageof 15
months and in 1994 a second MMR dose was in-
stituted for 6-year-olds. As a result, by the 1995–1998
period the average number of mumps cases had fallen
to 367 annually and the number of cases/100000
population had fallen to between 0 and 3 .
However, in September 2009 and continuing for
12 months Israel experienced its largest mumps
outbreak since 1988, with over 5000 reported cases
and a monthly incidence rate that peaked at 13.3
* Author for correspondence: E. Anis, MD, MPH, Director,
Division of Epidemiology, Ministry of Health, Israel.
Epidemiol. Infect., Page 1 of 8.
f Cambridge University Press 2011
cases/100000 population. In this mumps outbreak, as
had also been the case in Israel’s 2007–2008 measles
outbreak , lower vaccination rates in some popu-
lation subgroups played an important role. However,
in marked contrast with the measles outbreak, in
which over half the patients were aged <10 years and
in which only 4.6% had been fully vaccinated for
their age, the mumps outbreak had a greater impact
on older age cohorts and on individuals who had been
fully vaccinated – epidemiological features similar to
those in recent mumps outbreaks in Europe and
North America [4–8].
This study discusses the epidemiology of the
2009–2010 mumps outbreak in Israel in the context of
recent outbreaks elsewhere; reviews the major ap-
proaches to the effectiveness issues that have emerged
regarding the mumps component of the MMR vac-
cine; and considers the implications for public health
BACKGROUND AND METHODS
Mumps has been notifiable by law in Israel since 1977.
Cases are reported to the Health Ministry’s district
offices by physicians and medical laboratories, and
national data are processed and analysed by the
Ministry’s Division of Epidemiology. Individual case-
notification reports indicate age, gender, nationality,
address, date of onset, laboratory diagnostic test re-
sults, and the patient’s prior vaccination history. The
reports are based on epidemiological investigations
conducted by district health office staff. The data
generated from these case reports are used to track the
course of infectious disease outbreaks and to monitor
key epidemiological indicators. All quantitative data
regarding the 2009–2010 mumps outbreak in Israel
are based on this information system. Other epidemi-
ological assessments, such as the extent of infection
in population subgroups or the relative vaccination
coverage of these groups, derive from a familiarity
with the demographic characteristics of the localities
from which the case notifications are received, and
from ongoing communication with professional staff
in local health clinics and district health offices.
The clinical case definition used to detect cases for
reporting purposes describes mumps as a self-limiting
acute illness characterized by the swelling of a parotid
or other salivary gland lasting o2 days, with epi-
demiological linkage to other cases. Laboratory
confirmation is not required for routine reporting
purposes; during the 2009–2010 outbreak 704 cases
were laboratory-confirmed (13% of total reported
cases). Serological tests for specific IgG and IgM
antibodies, and virus detection using RT–PCR,
are conducted for verification purposes. Nucleic acid
sequencing and genotyping of the virus strain are
performed in selected cases. As is common in passive
surveillance systems the extent of underdiagnosis
and underreporting is unknown. Since notification
methods have remained unchanged over the years,
incidence data are considered indicative of actual
The mumps vaccine that has always been used in
Israel is the attenuated Jeryl Lynn strain, which is
included in the MMR vaccine (M-M-R II, Merck,
and Priorix, SKB) . Vaccination coverage estimates
for the first MMR dose are based on a representative
sample of children born in each health district and
registered in Mother and Child Health Services
clinics, which provide care to pre-school children.
Vaccination coverage data for the second dose, which
is provided to 6-year-olds in the first grade, are sub-
mitted to the Health Ministry by the School Health
Services. Since 1991 the first-dose coverage rate has
ranged from 94% to 96%. Coverage data for the
second dose are available for school years 2002–2003
to 2009–2010 and have ranged from 90% to 97%,
averaging 94% over the period. Although these
summary coverage rates might appear high enough to
limit the spread of mumps once it had been in-
troduced, Israel has always been plagued by relatively
poor immunization compliance by certain subgroups
within its ultra-orthodox Jewish community, a sector
that comprises about 10% of the total population.
Accurate compliance data for this subpopulation are
unavailable, but it is estimated to be 5–15% lower
than the national average for the first MMR dose.
School health service records for the ultra-orthodox
population are incomplete, and it can be assumed that
fewer 6-year-olds in this sector receive the second
MMR dose than do those in the general population.
Course of the outbreak
According to an epidemiological investigation the
2009–2010 mumps outbreak in Israel had its origin in
a visit to Jerusalem by students from a New York
yeshiva (religious boarding school). The students had
been infected during a New York-area outbreak that
originated at an orthodox Jewish summer camp; the
2E. Anis and others
virus had been imported there from England, where a
mumps outbreak had begun earlier that same year
[9–11]. During the first 10 weeks of the Israeli out-
break 173 cases of mumps were reported in the
students. In the initial stages of the outbreak three-
fifths of case patients were aged between 10 and
19 years; the median age was 15 years. Two thirds of
these patients reported having received two doses
of the MMR vaccine.  The Central Virology
Laboratory of the Ministry’s Public Health Services
identified the virus in circulation as genotype G5, the
same genotype identified in the 2009 outbreak in
The populous Jerusalem yeshiva community con-
tinued to be the centre of mumps infection as the
scope of the Israeli outbreak grew. Over time, how-
ever, the disease gradually spread outward into the
community at large. As the outbreak moved from
the older, male yeshiva population into the local
community, a gradual shift to the left occurred in
age-group incidence, along with a reduction in the
male-to-female case ratio. From autumn 2009 to
spring and summer 2010 the percentage of case
patients aged o10 years fell from 77% to 59%, and
the percentage of male patients fell from 78% to 59%
(see Table 1).
After rising steadily during autumn and early
winter of 2009–2010, monthly incidence peaked in
had declined significantly
September 2009 to August 2010 the number of
mumps cases throughout Israel totalled 5239, half of
which occurred in the Jerusalem health district. The
outbreak was limited almost entirely to the Jewish
population (over 98% of case patients were Jewish)
and remained highest in the ultraorthodox com-
munity, primarily among male yeshiva students. Over
the 12-month period two thirds of case patients were
aged >10 years, nearly half were between 10 and 19,
Table 1. Age and sex of case patients by stage of outbreak
Age o10 years
Age <10 years
Values are percentages.
Stage 1: 1 September 2009–31 December 2009.
Stage 2: 1 January 2010–31 March 2010.
Stage 3: 1 April 2010–31 August 2010.
Source: Individual case-notification reports.
Fig. 1. Epidemiological curve of the 2009–2010 mumps outbreak in Israel., Reported date; –&–, date of onset.
Mumps outbreak in Israel3
and nearly 40% were aged o15 years (see Table 2).
Sixty-four per cent of case patients were male. Of the
64% of patients whose vaccination status was known
78% had been fully vaccinated for their age. Sixty-six
patients were hospitalized, and there were no fatal-
ities. Reported complications included 14 cases of
orchitis, four cases of epididymo-orchitis, five cases
of meningitis, four cases of meningoencephalitis, two
cases of hearing loss in one ear, two abortions, one
case of pancreatitis, one case of labyrinthitis, and one
oedema of the sternum.
The Ministry of Health instructed community pro-
fessional staff to continue implementing standing
orders regarding mumps prevention: to maintain
vaccination coverage at high levels; to ensure the
timeliness of vaccination, particularly for 12-month-
old infants and first-grade pupils; to target those with
incomplete immunization histories, particularly chil-
dren and healthcare workers; to alert district office
staff to the possibility of new mumps cases and to
confirm and report incident cases. Vaccination catch-
up operations were implemented wherever mumps
was occurring and a second dose was offered in
kindergartens, schools, religious academies, and in
families to persons who had not previously received
two doses of mumps-containing vaccine. A third dose
of vaccine, or the measuring of antibody levels of
those previously vaccinated, was not recommended.
Additional recommendations included exclusion
from school and child care for 5 days from the onset
of parotid gland swelling, and droplet precautions in
hospitalized patients until 5 days after the onset of
parotid gland swelling. Immunization of all contacts
with one dose of mumps-containing vaccine, regard-
less of vaccination status, was considered but not im-
plemented due to a temporary vaccine shortage. In
all, a few thousand extra vaccine doses were given in
defined outbreak settings. After the outbreak peaked
in February and fewer new cases were observed each
month, it appeared that the minority of young adults
with low immunity had acquired mumps naturally or,
in some cases, by immunization.
Based on long experience working with the ultra-
orthodox community in Israel we hypothesize that the
high incidence of mumps among yeshiva students can
be linked to lower levels of immunization coverage
among some ultra-orthodox sectors, and to the high-
density congregate living environment, featuring long
hours of face-to-face study in crowded study halls,
that characterizes yeshiva life. While all yeshiva stu-
dents are male and reside in dormitories or communal
apartments, there were ample opportunities for the
disease to eventually spread outwards into the com-
munity at large. Students with mumps are generally
sent home to recover, where they can infect their
younger siblings, i.e. boys and girls who attend com-
year is 12 months long, there are three vacations of
3 weeks’ duration, corresponding to major religious
holidays, when students incubating mumps can infect
other family members and friends in their home
Table 2. Age group incidence distribution by stage of outbreak
0 1–4 5–910–14 15–1920–24 25–29
Values are percentages.
Stage 1: 1 September 2009–31 December 2009.
Stage 2: 1 January 2010–31 March 2010.
Stage 3: 1 April 2010–31 August 2010.
Source: Individual case-notification reports.
4E. Anis and others
Two epidemiological features of the 2009–2010
mumps outbreak in Israel are of special note: the
relatively high incidence among older age cohorts
during much of the outbreak, and the high rate of
vaccination coverage among case patients. Prior to
the vaccine era those most susceptible to mumps were
children between the ages of 2 and 12 years , and
today in countries that do not vaccinate against
mumps incidence is highest in children aged from 5 to
9 years [14, 15]. In the recent mumps outbreak in
Israel, however, the virus infected a comparatively
higher proportion of adolescents and young adults,
and the most salient feature of the outbreak was the
high rate (78%) of mumps patients who had been
fully vaccinated for their age.
The shift to the left in age-group incidence, which
continued as the outbreak progressed, was probably
the consequence of the undervaccination of children
in some sectors of the Jewish ultra-orthodox com-
munity. As with the measles outbreak in Israel 2 years
earlier  this problem demonstrated how the clus-
tering of individuals with relatively low immunization
coverage can undermine the benefits of herd immun-
ity in a highly vaccinated society . Mumps sero-
positivity rates were also found to be lower in this
community just prior to the 2009–2010 outbreak.
Rates averaged 51.8% in the ultra-orthodox popu-
lation in Jerusalem compared to 68.1% for the gen-
eral population and, for those aged o10 years, 46%
compared to 70% (D. Cohen, Tel Aviv University,
unpublished data). As with the 2007–2008 measles
outbreak, the undervaccination problem facilitated
the spread of the virus in certain subgroups. But what
was different about the mumps outbreak was how
diverged from earlier experiences with vaccine-
preventable childhood diseases.
Similar epidemiological trends have been appearing
in recent mumps outbreaks elsewhere. In a major
outbreak in the UK in 2005 the majority affected were
college and university students between the ages of 19
and 23 years. Close to one third of these patients had
received mumps vaccinations, although many were
either unvaccinated or had been immunized only once
before the two-dose MMR regimen had become
routine [17–19]. During 2009 in England and Wales,
mumps incidence was most highly concentrated in the
15–24 years age group, primarily in university
students. Here too many case patients were under-
vaccinated because they were too old to have
benefited from the two-dose MMR schedule .
During a 2006 outbreak in Austria the highest inci-
dence rates occurred in those aged 18–30 years . In
a 2009–2010 mumps outbreak in The Netherlands
65% of case patients were students, the median age
was 21 years, and of those students whose vaccination
status was known 79% had received two mumps
vaccinations . At the epicentre of the 2006 mumps
outbreak in the USA where 85% of the cases were
clustered, incidence was highest in the 18–24 years age
group; of patients in this age group whose vaccination
histories were known the proportion of two-dose re-
cipients was 84% [6, 21]. In an 8-month period during
a 2009–2010 New York area outbreak, 97% of all
cases patients were orthodox Jewish yeshiva students
and 61% were aged between 7 and 18 years. Of those
in this age group with known vaccination histories,
75% had received two doses of MMR vaccine .
Several features of the recent mumps resurgence
highlight the transition from the pre-vaccine to the
post-vaccine era. Members of certain age groups
were too old to have benefited from the two-dose
MMR regimen, but too young to have gained im-
munity to mumps through natural infection during
childhood. Many of these individuals were of univer-
sity-student age in the past decade and were especially
vulnerable to the outbreaks that occurred; they have
been described collectively as a ‘lost cohort’ [5, 8, 22].
Some health professionals have failed to recognize
mumps symptoms when presented with them, delay-
ing the implementation of measures that might have
helped prevent the virus from spreading [5, 23]. With
the recent mumps resurgence numerous medical
practitioners have been reintroduced to the disease
or are seeing it for the first time. Further, the diag-
nosis of mumps has been found to be more difficult
in previously vaccinated patients than in naive
Given the large number of two-dose MMR re-
cipients who have been infected during recent out-
breaks, the efficacy of the mumps vaccine has been
called into question [4, 25, 26]. There are several
potential causes for vaccine failure  and their rel-
evance to the epidemiological features of recent
mumps outbreaks varies. While problems such as
primary vaccine failure, or errors in the handling
and storage of vaccines have always explained why
some vaccinees remain susceptible to mumps, they
fail to address the timing issue: Why has there been a
resurgence of mumps outbreaks in recent years?
A potential explanation that has been suggested is
that of genotype mismatch – the Jeryl Lynn strain,
Mumps outbreak in Israel5
which is used in the USA, Canada, and widely in
Europe, belongs to mumps virus genotype A, and the
major outbreaks of recent years have been caused by
virus genotype G [13, 27]. It seems unlikely, however,
that genotype mismatch is a major factor in these
outbreaks. The Jeryl Lynn strain has been found to
be both protectiveof
and reasonably effective under epidemic conditions
[21, 28]. In Israel’s recent outbreak, for instance, the
genotype G virus did not spread to the general popu-
lation, which is protected bythe Jeryl Lynn strain.In a
recent serological study, the Jeryl Lynn strain effec-
tively neutralized genotype G virus samples that had
been taken from patients in the 2006 outbreak in the
USA . Moreover, genotype mismatch cannot ex-
plain why suddenly, after years of experience to the
contrary, incidence should be lower in children than
in adolescents and young adults.
One feature of the mumps vaccine that could ac-
count for recently changing patterns in both age-
group incidence and patient vaccination history is the
waning immunity factor. Some studies had identified
primary vaccine failure as the key reason for mumps
infection during the vaccine era , but when mumps
outbreaks began affecting large numbers of fully vac-
cinated adolescents and young adults, the waning
immunity issue began receiving more attention [5, 26,
27]. Several studies conducted in the USA found that
the risk of contracting mumps was correlated with the
number of years since last having received a dose of
vaccine . In one university that had been at the
epicentre of the 2006 outbreak, and at which two-dose
mumps were found to have increased in students aged
18–19 years who had been vaccinated o10 years
earlier . In England, waning immunity was found
to have occurred in previously vaccinated children
who were infected during the 2004–2005 outbreak in
that country .
Aside from the recent epidemiological record, lab-
oratory tests have shown a decline in seropositivity
rates and antibody titres following vaccination, in-
dicating a degree of waning immunity that could place
older vaccinees at risk during an outbreak .
Antibody decay was discovered in tests among uni-
versity students and staff between the ages of 19 and
30 years who had received two MMR doses .
Further, a recent study of mumps seroprevalence in
population groups in the USA raised similar concerns
about borderline herd immunity levels . As Dayan
et al. observed, even at high (95%) vaccination
the general population
90% protection rate comes precariously close to the
estimated herd immunity threshold .
High-density environments increase the risk of
contagion, and the clustering of susceptible in-
dividuals has brought mumps prominently to the
campus. In societies where MMR vaccination had
become routine, mumps incidence has shifted from
children in primary school to adolescents and young
adults – particularly those in colleges and universities.
These institutions bring together large numbers of
individuals who, by virtue of their age, may be more
susceptible to the virus, and mumps incidence in such
settings has become more widespread as a result
[6, 21, 28].
The changing profile of the typical mumps patient
suggests that public health officials need to consider
new strategies for combating the virus.
Given the increased risk of mumps infection in
older age cohorts and the related risk of outbreaks
in high-density environments, analysts have rec-
ommended that the clustering of susceptible ado-
lescents and young adults be avoided. Since persons in
these age groups tend to enter universities and the
military in large numbers, the provision of booster
vaccinations to all susceptible individuals entering
these congregate living environments – including both
the unvaccinated and those whose immunity may
have weakened since their last vaccination – could
prove a highly effective outbreak prevention measure
[27, 31, 33].
A large-scale (albeit unintentional) clinical trial
may have bolstered the argument for just such
a booster vaccination programme. During 2006
American soldiers should have been as susceptible
to that year’s US mumps outbreak as were their
university counterparts. Military personnel live in
high-density environments; recruits belong to the
university-student age cohort that was central to
the 2006 outbreak; and large numbers of soldiers are
stationed on bases in the midwestern region of
the country where the outbreak was concentrated.
Military policy, however, was that all recruits without
documentation of prior vaccination or other evidence
of immunity to measles and rubella, receive a dose of
MMR vaccine. Consequently booster vaccinations
(and in an unknown number of cases a third dose of
MMR vaccine) were provided to a large population of
potentially susceptible individuals. The result was
that, of a total population of nearly 1.4 million per-
sonnel, only 53 cases of mumps were reported in the
and effectivenesslevels,the resulting
6E. Anis and others
US military during 2006 – a level that fell within the
military’s 10-year range of aggregate mumps inci-
Today, most children and most adults are protected
against mumps – the former group because they have
been vaccinated fairly recently, the latter group
because they are old enough to have gained natural
era. But a new class of susceptibles has emerged:
adolescents and young adults with no natural mumps
immunity and whose vaccine-based immunity is dis-
appearing. That this same cohort populates a multi-
tude of congregate living environments greatly
heightens the risk of future, large mumps outbreaks.
Booster vaccinations for those at high risk of infection
during mumps outbreaks, such as yeshiva and uni-
versity students, merit priority consideration.
We acknowledge with thanks the Ministry of Health
Advisory Committee on Immunization and Infectious
Diseases; the National Center for Mumps, Measles
and Rubella at the Central Virology Laboratory of
the Ministry’s Public Health Services; the staff of the
Jerusalem Health District Office and other health
district offices for their assistance in controlling the
outbreak; and Mr Ruslan Gosinov for his assistance
with data management and his work on the graphics.
DECLARATION OF INTEREST
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