Rate and time distribution of relapses in multibacillary leprosy.

Of the 47,068 patients registered in the Polambakkam Leprosy Center between 1955 and 1982, we selected 1886 cases having shown bacteriological positivity at any time during this period, whatever their classifications at registration, and subsequently found bacteriologically negative. After an average follow-up period of 10 years, 243 relapses were observed, giving a crude relapse rate of 12.8 per person-years of observation and a cumulative probability of relapse of 18.9%. Relapse rates were found to be dependent on regularity during smear-positive and -negative periods; a regularity greater than 75% in the smear-positive period proved to be particularly important. The results show no evidence that relapses occurring after 3 years of negativity could be reinfections, and that the relapse rate was still affected by regularity 7 years after negativation. The median delay of relapses was found to be 4.4 years and was not affected by the regularity of treatment.