Two-Year Duration of Immunity of Inactivated Poliovirus Vaccine: A Follow-up Study in Pakistan in 2020.

This was a follow-up study conducted in 2020 assessing changes in levels of type 2 poliovirus-neutralizing antibodies 2 years postimmunization in children who received inactivated poliovirus vaccine (IPV) in Karachi, Pakistan. Unexpectedly, the findings revealed an increase in seroprevalence of type 2 antibodies from 73.1% to 81.6% 1 year and 2 years after IPV, respectively. The increase in type 2 immunity could result from the intensive transmission of circulating vaccine-derived poliovirus type 2 (cVDPV2) in Karachi during the second year of IPV administration. This study suggests that the cVDPV2 outbreak detected in Pakistan infected large proportions of children in Karachi. Clinical Trials Registration . NCT03286803.