From May 29 to October 26, 1982, 1,031 cases of paralytic
poliomyelitis were reported to the Taiwan health authorities, for
an
overall attack rate of 5.8 per 100,000 population (Figure 7). This
was the largest poliomyelitis outbreak in Taiwan's history (1).
Patients ranged in age from 2 weeks to 31 years (median 16
months). Of the 1,031 patients, 646 (63%) were less than 2 years
of
age and 189 were 2-5 years of age, yielding age-specific attack
rates
of 82.2/100,000 and 12.0/100,000, respectively. Ninety-five (9%)
persons with polio died. Attack rates varied by geographic area
and
ranged from a low of 1.3/100,000 population in Tainan County to a
high
of 15.2/100,000 population in Yun Lin County. Type 1 poliovirus
has
been isolated from 247 (46%) of 537 specimens submitted from
patients
during the outbreak. Vaccination status was known for 881 (86%)
patients. Discounting doses of oral polio vaccine (OPV) received
within 28 days of onset of illness, 65% of these had received no
prior
polio vaccine; 19% had received one dose; 8% had received two
doses;
and 8% had received three or more doses.
From 1975 through 1981, Taiwan was free of major poliomyelitis
outbreaks. Estimates by Taiwan health authorities of vaccination
levels in 1981 were that, by the first birthday, approximately 80%
of
infants had received at least two doses of trivalent OPV. During
the
outbreak, assessments of immunization levels were conducted using
the
World Health Organization's Expanded Program on Immunization
cluster
sampling method* in each of four counties and two cities (2).
Assessments of immunization levels for children 12-35 months of age
demonstrated that 83%-98% of the 12- to 35-month-old children had
received two or more doses of OPV in the six areas before the
outbreak. Less than 7% of the surveyed population had not received
any doses of OPV.
A mass vaccination program was undertaken by health authorities
utilizing OPV. Initially, the program was directed at children
under
5 years of age and, subsequently, at those up to 15 years of age
(through junior high school age). Following this mass vaccination
program, assessments of immunization levels in these six areas
indicated that more than 50% of 12- to 35-month-old children
received
at least one dose of OPV during the control program, resulting in a
coverage of 91%-99% for two or more doses of OPV. An average of
59%
5 years of age and, subsequently, at those up to 15 years of age
(through junior high school age). Following this mass vaccination
program, assessments of immunization levels in these six areas
indicated that more than 50% of 12- to 35-month-old children
received
at least one dose of OPV during the control program, resulting in a
coverage of 91%-99% for two or more doses of OPV. An average of
59%
of surveyed children with no prior history of polio vaccine
received
at least one dose during the mass campaign.
Yun Lin and Chia Yi counties were studied to determine risk
factors for disease. These counties were selected because they
represented contiguous rural areas with markedly different attack
rates, 15.2/100,000 and 2.5/100,000 population, respectively.
Households of non-cases in the cluster sample and households of
cases
in these counties were assessed for vaccination status of children
12-35 months of age, crowding, socioeconomic status, level of
sanitation, and relocation from their official household
registration
address (persons who moved would be less likely to receive reminder
notification for vaccination). Patients had had markedly fewer
vaccinations. Vaccine efficacy was estimated to average 82%
following
a single dose, 96% following two doses, and 98% following three or
more doses. In addition, the case households tended to have
significantly more young children, be more crowded and less
educated,
have a greater proportion of fathers who were either unemployed or
employed as unskilled laborers, live at addresses other than their
household registration, use non-municipal sewage disposal and
water,
and routinely share toilet facilities with other families.
Comparison
of non-case households in the two counties, however, demonstrated
no
significant differences in risk factors that could explain the
difference in attack rate between the counties.
Reported by TC Hsu, Director-General, ST Hsu, Director, Bureau of
Disease Control, KH Hsu, Div Chief, CI Ma, Div Chief, HM Hsu, Div
Chief, Taipei, Taiwan; Div of Viral Diseases, Center for Infectious
Diseases, Div of Immunization, Center for Prevention Svcs, CDC.
Editorial Note
Editorial Note: Failure to vaccinate rather than vaccine failure
represented the most important risk factor for paralytic
poliomyelitis. Ensuring that children are vaccinated at the
earliest
recommended age offers the greatest chance for protection. Data
analyses are ongoing to determine whether the other risk factors
contributed independently to risk of disease or simply correlated
with
failure to receive vaccine.
This outbreak demonstrates that major epidemics can occur in
areas
that have not had substantial poliomyelitis activity for many years
and that have relatively high overall community vaccination levels.
Unrecognized clusters of susceptibles can exist and may be
sufficient
to sustain transmission of wild poliovirus in a community. On a
smaller scale, such clusters were responsible for the last two
polio
outbreaks in the United States in 1972 and 1979 and in the
Netherlands
and Canada in 1978 among religious groups declining vaccination
(3).
Identification and vaccination of subpopulations with low coverage
is
essential to controlling poliomyelitis.
References:
CDC. Poliomyelitis--Taiwan. MMWR 1982; 31; 493.
Henderson RH, Sundaresan T: Cluster sampling to assess
immunization coverage: a review of experience with a
simplified
sampling method. Bull WHO, 1982;60:253-60.
CDC. Poliomyelitis surveillance summary 1979, Issued April
1981.
Disclaimer
All MMWR HTML documents published before January 1993 are electronic conversions from ASCII text into HTML. This conversion may have resulted in character translation or format errors in the HTML version. Users should not rely on this HTML document, but are referred to the original MMWR paper copy for the official text, figures, and tables. An original paper copy of this issue can be obtained from the Superintendent of Documents, U.S. Government Printing Office (GPO), Washington, DC 20402-9371; telephone: (202) 512-1800. Contact GPO for current prices.
**Questions or messages regarding errors in formatting should be addressed to mmwrq@cdc.gov.
