Immunogenicity of type 2 novel oral poliovirus vaccine administered in outbreak response, Kenya 2023: an observational cohort study.

BACKGROUND: In 2023, novel oral polio vaccine type 2 (nOPV2) was used to respond to an outbreak of circulating vaccine-derived poliovirus type 2 (cVDPV2) in Kenya. We assessed the immunogenicity of one and two doses of nOPV2 administered during outbreak response. METHODS: An observational cohort study was conducted from October to December 2023 in Kitui and Tana River counties in Kenya. Children aged 6-59 months were enrolled. Two nOPV2 doses were administered. Dried blood spots (DBS) were collected at enrollment, four-weeks after the first nOPV2 dose, and two- and four-weeks after the second nOPV2 dose. The DBS cards were tested at the US Centers for Disease Control and Prevention (CDC), Atlanta for the presence of poliovirus neutralizing antibodies using microneutralization assays. The primary endpoint was cumulative immune response after two nOPV2 doses. FINDINGS: Of the 475 participants followed in the study, 461 (97%) completed all study visits and were included in the analysis. Four weeks after two nOPV2 doses, a cumulative type 2 immune response was observed in 285 (62%; 95% CI 57-66) of 461 participants. After one nOPV2 dose, type 2 immune response was observed in 196 (43%, 95% CI 38-47) of 461 participants. Type 2 seropositivity (titers ≥1:8) increased from 19% at enrollment to 72% after two nOPV2 doses. INTERPRETATION: The study demonstrated an immune response in 62% of the participants following two nOPV2 outbreak vaccination rounds. These findings suggest that two nOPV2 doses may be insufficient to achieve high population immunity during poliovirus type 2 outbreak response in sub-Saharan Africa.