IgM and IgG responses for normal donors, LF and NL febrile subjects. Box plots of LASV-specific IgM (A) and IgG (B) levels determined by ELISA, are displayed as mean OD450 with corrected cutoff values based on the 95th percentile of established negative control sera. Each display shows the minimum non-outlying value, three quartiles, maximum non-outlying value, and outlying values. The comparison groups include U.S. normals (US N), Moyamba district normals (MOY NHS), Bombali district normals (BOM NHS), convalescent LF follow-up patients (between 8 and 108 weeks post discharge [LF FU]), nonfatal acute LF cases (LF NF), fatal LF cases (LF F), and non-Lassa febrile illness with LASV-specific IgM only (NL FI IgM+). IgM and IgG levels for patients in the LF FU sera group were significantly higher than those in for all of the other comparison groups, save for those in the BOM NHS and NL FI IgM+ cohorts. There were no significant differences between LF NF and LF F cases for both IgM and IgG responses. Bombali sera showed relatively high levels of LASV-specific IgM and IgG, but these levels did not significantly differ from those for LF FU patients, despite their undiagnosed recent LF or other febrile illnesses. Outliers are indicated with red asterisks (*). Significant p values for pairwise comparisons are displayed as * p < 0.05; ** p < 0.01; and *** p < 0.001.