BackgroundSeroprevalence studies can indicate the age-distribution of first infection with non-typhoidal Salmonella (NTS) and estimate the rate of infection by age.MethodWe collected 1254 paired samples of serum and stool from healthy children and adults in Kilifi, Nairobi, and Siaya counties in Kenya, areas of low, medium and high incidence of invasive NTS disease (iNTS), respectively. We quantified the age and site-specific geometric mean concentrations (GMCs) of IgG and IgA antibodies against S. Enteritidis O-antigen (O:9, serogroup D) and S. Typhimurium O-antigen (O:4,5, serogroup B) using an in-house standardised ELISA. Serum concentrations were estimated against a previously calibrated reference serum.ResultsMaternally derived O:9 IgG and O:4,5 IgG antibodies were detectable in 100% of neonates and the GMC decreased by 40% (95%CI [25%-52%]) per month in the first six months of life. GMCs of IgA were low in neonates. After age 6 months, the O:9 and O:4,5 IgG and IgA GMCs increased sharply with age across all sites, reaching a plateau in early adulthood. The rate of increase in IgG GMCs by age was highest for O:9 in Nairobi, and for O:4,5 in Kilifi. Mixture modelling defined a threshold of 14.1 AU for O:9 IgG and 28.2 AU for O:4,5 IgG. Seroprevalence also increased by age. The GMCs of O:4,5 IgG were 2 times higher for carriers of serogroup B Salmonella than non-carriers.DiscussionMaternal antibodies to non-typhoidal Salmonella decay rapidly from 0-5months after which incident infection with both serogroups occurs. To be effective, control efforts should be implemented before primary infection.