These data suggest that (1) most symptomatic and acute LF patients presenting to KGH LFW are naïve to LASV; (2) exposure to LASV in Sierra Leone is more prevalent than previously reported and remains mostly undiagnosed; (3) regions of Sierra Leone previously considered non-endemic for LF revealed a significant level of LASV-specific IgM and IgG prevalence. Here, CD40LG is linked to Lassa fever.