Circulating Vaccine-Derived Poliovirus Type 2 Outbreak in Sierra Leone, December 2020: Analysis of the Initial Three Cases

Poliomyelitis remains a critical global public health concern despite major progress toward eradication. Following the 1988 World Health Assembly resolution for worldwide polio eradication, the Global Polio Eradication Initiative (GPEI) has reduced endemic countries to Afghanistan and Pakistan and decreased wild polio cases to single digits in these regions. However, circulating vaccine-derived poliovirus type 2 (cVDPV2) continues to pose challenges in countries with suboptimal immunization coverage. This study investigated the first three confirmed cVDPV2 cases in Sierra Leone in December 2020, assessing vaccination history, case characteristics, and potential transmission sources. A case investigation was conducted using the WHO zero-dose case investigation form, and a vaccination coverage survey was performed in 128 randomly selected households within the affected communities. Cold chain management and routine immunization services were also evaluated to identify gaps contributing to the outbreak. The three case-patients were a 26-month-old male from Kambia, a 39-month-old female from Western Area Rural, and a 15-month-old female from Tonkolili district. All presented with fever and acute flaccid paralysis. None had a history of travel within 21 days prior to symptom onset. Two cases were genetically linked to poliovirus strains previously reported in Guinea and Côte d’Ivoire. All children had received three doses of oral polio vaccine (OPV), with two children also receiving one dose of inactivated polio vaccine (IPV). Vaccination coverage in the affected communities revealed gaps in routine immunization, while evaluation of cold chain management highlighted challenges in maintaining vaccine potency. These findings underscore the risk of cVDPV2 emergence even in regions with high OPV coverage and highlight the importance of robust surveillance, cold chain maintenance, and timely vaccination campaigns. Strengthening routine immunization, improving outbreak response strategies, and ensuring adherence to vaccination protocols are essential to prevent future cVDPV2 outbreaks and sustain progress toward polio eradication.