The effect of intervals between surveys on the estimation of incidence rates of leprosy

Abstract

This paper examines the effect that variation in the interval between successive cross-sectional surveys may have on estimates of the incidence rates of leprosy. The results of the present study showed that when surveys of the contacts of leprosy patients were conducted in consecutive years (gap between surveys of 1 year) the estimated incidence rate of leprosy was 4.7 per 1000 person years of risk. When there was a gap of 3 years between surveys the estimated incidence rate of leprosy was only 1.9 per 1000 person years of risk. Thus when the between-survey interval increased from 1 to 3 years, the estimated incidence rate of leprosy was halved. Similar findings were obtained from the results of prevalence surveys in the general population. The implications of these findings in relation to survey work in leprosy and possible vaccine trials are discussed.