Low vaccination rates lead to global surge in vaccine-derived polio

Key takeaways:

• There was a 694% increase in paralytic cases of type 1 circulating vaccine-derived polio from 2021 to 2022.
• The new oral vaccine for type 2 poliovirus resulted in a 46% decrease in paralytic cases from that type.

Lower vaccination rates led to a global surge in cases of one type of vaccine-derived polio last year, according to a report published Thursday in MMWR.

Researchers said the increasing number of cases of circulating vaccine-derived poliovirus type 1 (cVDPV1) — as well as the ongoing transmission of cVDPV type 2 (cVDPV2), the type at the center of the outbreak reported last year in New York — threaten the goal to stop all circulating vaccine-derived polio by the end of 2023.

Outbreaks of cVDPV “can occur when oral poliovirus vaccine ... strains undergo prolonged circulation in undervaccinated populations, resulting in genetically reverted neurovirulent virus,” the authors of the new report noted.

According to the report, the proportion of cVDPV cases caused by cVDPV1 increased from 3% in 2020 to 18% in 2022 following “a substantial decrease in global routine immunization coverage and suspension of preventive immunization campaigns during the COVID-19 pandemic.”

“Outbreak responses in some countries were also suboptimal,” the authors wrote.

Cases of vaccine-derived polio occur when a live but weakened strain of virus that is included in vaccines spreads between people Like wild polio, it can cause paralysis.

After type 2 poliovirus was eradicated in the wild, the Sabin type 2 vaccine strain was removed from routine use in 2016 as more than 150 countries switched to a bivalent polio vaccine containing only types 1 and 3.

A more stable monovalent type 2 poliovirus vaccine has been used to respond to vaccine-derived outbreaks from that type. According to the authors of the new report, the introduction of this vaccine is likely responsible for a marked decrease in new cVDPV2 emergences and a 46% decrease in the number of paralytic polio cases from this type between 2020 and 2022.

However, “Whereas the number of cVDPV2 emergences has declined during the 2021-2022 COVID-19 pandemic and recovery period, international spread has not,” the authors wrote. “During the last 2 years, 17 countries have experienced their first post-switch cVDPV2 outbreaks, reflecting poor outbreak control in the country of origin.”

The researchers noted a surge in cVDPV1 cases from 2021 to 2022, including a 694% increase in paralytic cases from this type to 127, with most of the 14 outbreaks since 2016 occurring in the Democratic Republic of the Congo, Madagascar, Malawi, Mozambique and Yemen.

“Improving routine immunization coverage, strengthening poliovirus surveillance, and conducting timely and high-quality supplementary immunization activities in response to cVDPV outbreaks are needed to interrupt cVDPV transmission and reach the goal of no cVDPV isolations in 2024,” they wrote.