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

Abstract

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.