Poliomyelitis surveillance and vaccine efficacy in Bombay, 1982-87.

Poliomyelitis surveillance and vaccine efficacy in Bombay, 1982-87 Introduction A report has previously appeared on poliomyelitis surveillance and vaccine efficacy in Bombay, determined from data in the Annual Reports of the Enterovirus Research Centre (ERC) of the Indian Council of Medical Research for the period 1949-82 [1]. For Greater Bombay, the estimated average efficacy of trivalent oral poliovaccine (OPV) was 86% for the period 1979-81. In 1985 Bombay was designated as a site for the global "local area monitoring" network of the WHO Expanded Programme on Immunization (EPI), and since then data on population statistics and immunization coverage have been collected from the Public Health Department of the Bombay Municipal Corporation (BMC). Surveillance for poliomyelitis is conducted by the ERC. A description of the local area monitoring programme in Bombay for the period 1974-84 has already appeared [2]. The current OPV immunization schedule for children in Bombay is the same as that used in the rest of India and allows the first dose to be administered to children aged as young as 6 weeks, followed by second and third doses at intervals of at least 4 weeks, respectively, and a supplemental dose at 18 months of age. Prior to 1987 the immunization schedule recommended that OPV be administered to children aged 3, 4, 5, and 18 months. Here, we present updated poliomyelitis surveillance data for the period 1982-87 that were obtained from the local area monitoring programme and also provide more precise estimates of the effectiveness of OPV in Greater Bombay. Method A case of poliomyelitis was defined as a patient who had been diagnosed by a physician to have paralytic poliomyelitis. Patients were considered to be residents of Bombay if their normal address was within Greater Bombay. To identify cases, the ERC health worker visits at least once each week the four principle hospitals in Bombay that admit children with poliomyelitis: Kasturba Infectious Disease Hospital, B.J. Children's Hospital, J.W. Children's Hospital, and L.T.M.G. Hospital. The ERC also makes efforts to detect cases that are admitted to other public or private hospitals. Periodic visits to the major rehabilitation centre in Bombay helps to detect nonhospitalized cases. Information on cases is recorded on standardized forms based on data obtained by the ERC health worker from hospital records and by interviewing the parents of patients. The immunization status of patients is obtained from data on immunization cards or records or from parental histories if no records are available. Poliomyelitis immunization coverage in Bombay is estimated from reports of the number of OPV doses administered and from the results of standard EPI 30-cluster evaluation surveys. (a) Vaccine efficacy (VE) is calculated using the formulata [3, 4]: VE=(Poliomyelitis attack rate (unimmunized)-Poliomyelitis attack rate (fully immunized))/Poliomyelitis attack rate (unimmunized) This formula can be modified to determine the VE of multiple does vaccines by including terms for the vaccination status of cases and the population. The resulting formula, which was used to calculate VE in the present study, is given by [5]: VE=(PPV)(PCU)-(PPU)(PCV)/(PPV)(PCU) where PCU=proportion of cases unvaccinated against poliomyelitis; PPU=proportion of the population unvaccinated; PCV=proportion of cases vaccinated with the number of doses being examined for vaccine efficacy; and PPV=proportion of the population vaccinated with the number of doses being examined for vaccine efficacy. The VE was estimated using the case exposure method [4], as follows: by analysing data from all resident cases and assuming that immunization coverage levels for all age groups in a given year are the same as those obtained from the reported number of doses of OPV administered to children aged under 1 year; and by separately analysing data from a subset of cases for the same area, age group, and year for which information was available from an immunization coverage survey. …