Strategies and lessons learned from a type 1 circulating vaccine-derived poliovirus (cVDPV) outbreak affecting adult population: Madagascar, 2020–2023

INTRODUCTION: From 2020 to 2023, Madagascar experienced an outbreak of circulating vaccine-derived poliovirus type 1 (cVDPV1) that notably affected adults and was associated with high mortality among paralytic cases. This atypical epidemiological pattern required adapted surveillance approaches and tailored outbreak response strategies, including vaccination campaigns targeting adult populations. METHODS: We conducted a descriptive analysis of the cVDPV1 outbreak in Madagascar between September 2020 and September 2023. Epidemiological data on confirmed and polio-compatible cases were reviewed, including age distribution, clinical presentation, and outcomes. We also documented outbreak response strategies, particularly vaccination campaigns and operational innovations implemented to reach adult populations. RESULTS: A total of 54 confirmed cases of paralytic polio caused by cVDPV1 were reported during the study period, including two adults presenting with fever and paralysis. Between February 2022 and March 2023, 15 adult acute flaccid paralysis cases were classified as polio-compatible by the National Polio Eradication Committee due to missing (86%) or negative laboratory specimens. Mortality among these compatible adult cases was 56%. In response, Madagascar conducted vaccination campaigns using bivalent oral polio vaccine, targeting the entire population in four regions and children under 15 years in the rest of the country. Critical role of country leadership, strategic mobilization, and effective communication in driving success and influencing risk perception during the 2023 campaigns helped achieve high coverage among adults. CONCLUSIONS: The cVDPV1 outbreak in Madagascar demonstrated the potential for significant adult involvement and high mortality in polio outbreaks. High vaccination coverage combined with targeted innovations and strong community engagement contributed to successful outbreak control. These findings highlight the importance of inclusive vaccination strategies and tailored approaches in similar settings.