humoral immunity during the first 2 years of life in children from endemic areas.

is a leading cause of moderate-to-severe diarrhea, primarily affecting children in impoverished regions. Disease incidence is highest in toddlers 1-2 years of age. Acquisition of immunity over time reduces the risk of infection. However, the evolution of childhood immunity resulting from exposure and the host responses that mediate protection remain poorly understood. Gaining insight into the immunological features that develop over time and prevent subsequent infection is critical for guiding vaccine design. We examined the antigenic repertoire and magnitude of humoral immunity to in children < 2 years of age from three endemic areas-Bamako, Mali, and two sites (Afar and Tigray) in Ethiopia-using a qualified multiplex assay. Serum IgG and IgA specific to proteins (IpaB, IpaC, IpaD, IpaH, and VirG) and lipopolysaccharide (LPS) from 2a, 3a, 6, and were measured in three stratified age groups: 6-8, 12-17, and 18-24 months. The youngest group (6- to 8-month-olds) exhibited similar antibody profiles across all sites. By contrast, antibody levels in the 12- to 17- and 18- to 24-month-old age groups varied by both age and site, reflecting geographical differences in endemicity and exposure. Notably, most children in the 12- to 17-month-old age group had IgG levels below the adult protective thresholds identified in controlled human infection models, identifying a critical window of vulnerability that would require intervention. Our findings underscore the importance and value of serosurveillance in children from endemic regions to inform future vaccine rollout decisions.IMPORTANCE is a major cause of moderate-to-severe diarrhea, the most affected being young children from poor resource countries. species easily acquire antibiotic resistance, presenting a challenge to infection control. The development of vaccines for young children has been hindered by a lack of understanding of what constitutes protective immunity. Here, for the first time, we investigated the magnitude and specificity of humoral immunity evoked by natural exposure in children 6 months to 2 years old living in Mali and Ethiopia (-endemic areas) using a qualified multiplex assay. Antibody profiles varied with age and region, revealing epidemiological trends. Children 12-17 months old were identified as the most vulnerable to infection. Antibodies specific for conserved proteins were higher in older children, affirming their potential as vaccine candidates. serosurveillance is useful in guiding public health interventions.