Factors Associated with the Occurrence of Serious Adverse Events Following Immunization Following Administration of Nopv2 in Children Under Five Years of Age in Benin

This is the authors' abstract. We don't add key points for this paper.

Background: The new oral polio vaccine type 2 (nOPV2), authorized by WHO in November 2020, was deployed in Benin in 2025 as part of response campaigns against circulating vaccine-derived poliovirus type 2 (cVDPV2). The factors associated with serious adverse events following immunization (AEFI) reported after this campaign remain poorly documented in the French-speaking West African context. Method: We conducted a national case-control study that was conducted from August 2025 to November 2025 in the twelve departments of Benin. A total of 1067 children were included in the study. Cases (n=510) were children 0 to 5 years of age with severe AEFI following nOPV2 administration and controls (n=557) were nOPV2-vaccinated children without AEFI. The associated factors were searched for by multivariate logistic regression, after selection of candidate variables in univariate analysis. Results: In multivariate analysis, five independent determinants were identified. History of past malnutrition was associated with higher odds ratios of severe AEFI (aOR = 12.37; 95% CI: [1.51-18.10]; p = 0.019). Loss or unavailability of the vaccination record (ORa = 1.76; 95% CI: [1.11-2.78]; p = 0.016) and vaccine incompleteness (ORa = 1.77; 95% CI: [1.10-2.87]; p = 0.020) were also associated with higher odds. Conversely, the verification of the health record (ORa = 0.36; 95% CI: [0.26-0.50]; p < 0.001) and the question about the child's health status before vaccination (ORa = 0.64; 95% CI: [0.45-0.92]; p = 0.016) were associated with lower odds. Age, which was significant in univariate analysis, was no longer significant after adjustment (ORa = 1.01; 95% CI: [1.00-1.02]; p = 0.2). Conclusion: Our results suggest that the occurrence of severe AEFIs for nOPV2 in Benin is mainly associated with children's nutritional vulnerability, vaccination status and the quality of pre-vaccination practices. These associations do not allow a causal relationship to be established between the vaccine and the events observed.