SURVEY OF VACCINATION STATUS AND PAST SMALLPOX EXPERIENCE OF AN URBAN POPULATION OF WEST PAKISTAN

This paper reports the results of a survey of vaccination status and smallpox history undertaken in the city of Lahore West Pakistan in 1968-69 6 months before introduction of an eradication program. An aim of the survey was to develop reliable estimates of the immunization status of a typical urban population that could be used as a baseline for assessment of the subsequent eradication program. The sampling procedure used in this study was a 3-stage cluster sample of the population. This provided an acceptable degree of precision with a minimum utilization of field resources. 6952 (86%) individuals from the final enumerated sample of 8064 were examined and interviewed. Overall 90.3% of the population was found to have vaccination scars; there were no significant sex differences. On the basis of interviews 93.4% had a positive vaccination history although 24.4% had received only primary vaccination. An estimated 68.3% of the population had a history of vaccination or revaccination within the last 3 years and 80.2% within the last 10 years. Vaccination status was directly related to location of the city area; lower rates of reported vaccination were obtained in peripheral areas and agricultural neighborhoods. An estimated 4.7% of the population had a history of previous smallpox. When the 90.3% with visible evidence of successful immunization were combined with the 2.8% with a history of past smallpox and no vaccination scars an overall level of past experience with pox virus antigens of 93.1% was obtained. These results suggest a relatively high level of vaccination in the Lahore area. The persistence of endemic smallpox despite this achievement is attributed to the considerable numbers of younger children who are not vaccinated and the waning of immunity in older age groups.