To determine seroprevalence and quantitatively monitor anti-PGL-I IgM levels over time in the leprosy endemic areas where active case finding and SDDR-PEP administration were implemented during the PEOPLE trial, FSB samples from index cases (n = 702) as well as their household and neighbourhood contacts (n > 17,000) were obtained during field screenings. Here, CD40LG is linked to leprosy.