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Update: Poliomyelitis Outbreak -- Taiwan

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:

  1. CDC. Poliomyelitis--Taiwan. MMWR 1982; 31; 493.

  2. 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.

  3. CDC. Poliomyelitis surveillance summary 1979, Issued April 1981.

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