Determining Community Immunization Level with Examples for Poliomyelitis

Abstract

T HE IMMUNIZATION level of a community may be defined as the percentage of the population protected by natural and artificially induced immunity. The level is ultimately measured by the community's disease experience as evidenced by cases reported and otherwise discovered. This method of measurement is applicable to all communities, but it may be of little value when cases of disea,se are rare, as with smallpox, or infrequent, as with poliomyelitis. In Pennsylvania there were few cases of poliomyelitis in Berks, Lehigh, and Northampton Counties for 5 years before the mass vaccination programs of 1963, and only one case since (see table). However, there was a considerable potential as shown by sizable 1963 outbreaks in nearby Philadelphia and Cumberland Counties. Case investigations necessary to validate the diagnosis include clinical, serologic, and virological studies on the patients. Such studies in Cincinnati were reported by Sabin (1) and in Czechoslovakia by Skovranek (s), as ruling out the diagnosis of poliomyelitis during 1960 and 1961. Case attack rates will show whether differences exist between vaccinated and unvaccinated groups at one particular time and may show the influence of vaccination over a period of time. An example of a study at one point in time is Fendall's report on the epidemic in Kenya in 1960, with more than twice as many cases in the unvaccinated as in the vaccinated (3). Chin showed the decrease in case rates in Des Moines, Iowa, and Kansas City, Mo., from 1954 to 1959, concurrent with increased Salk vaccination (4). The immunization level may also be measured by detection of antibodies in a reipresentative sample of individuals as reported from South Africa (5). The most recent report describes a survey of children 1 through 4 years of age in Cleveland, Ohio, following an oral vaccination program in 1962. Titers were determined in vaccinated and unvaccinated children. Advances in laboratory methods may bring periodic sero-immunity surveys within the, capabilities of many health departments (6). Intestinal infection occurring naturally or produced by ingestion of vaccine may be demonstrated by identification of virus in patients and in sample surveys. Both were, done in Israel before and during an outbreak of poliomyelitis in 1961 (7). Spread of virus may be shown by its recovery from sewage as was done in Hillsborough County, Fla. (8).