Vaccine-Derived Polio Traced to the Novel Type 2 Oral Vaccine, 2021–2023

Circulating vaccine-derived poliovirus (cVDPV) outbreaks happen when the weakened virus in oral polio vaccine — most often the Sabin monovalent type 2 vaccine (mOPV2) — keeps circulating for a long time among undervaccinated people and genetically reverts to a form that can paralyze. A novel type 2 oral polio vaccine (nOPV2) was developed to lower that risk; in clinical trials it was less likely than mOPV2 to revert, and early field use showed limited genetic change. About 700 million doses have been given worldwide to fight type 2 outbreaks. This CDC report describes the type 2 emergences traced to nOPV2 from its rollout in March 2021 through September 7, 2023.

What surveillance found

Reviewing surveillance and laboratory data from the Global Polio Eradication Initiative, investigators identified seven cVDPV2 emergences of nOPV2 origin, detected between August 2021 and July 2023 in 61 paralytic cases and 39 sewage samples from six African countries: Burundi, the Central African Republic, the Democratic Republic of the Congo (DRC), Nigeria, Tanzania and Zambia.

The viruses differed only slightly from the vaccine strain — six to 16 nucleotide changes in the region coding for the VP1 capsid protein — which indicates they were caught relatively soon after vaccination.

Detections of cVDPV2 emergences linked to nOPV2 use, 2021–2023. CDC map from the report.

How far they spread

• Four emergences stayed within the part of the country where they arose, in the Central African Republic, DRC and Nigeria.
• Two spread to other areas of their home countries, the Central African Republic and DRC.
• One spread from DRC into neighboring countries. This emergence, RDC-SKV-1, the largest and most widespread, was first found in DRC's South Kivu province in September 2022 and has since been identified in six DRC provinces and in Burundi, Tanzania and Zambia, most recently in Tanzania in July 2023.

Three of the spreading emergences may still have been circulating, with their latest detections in July 2023.

How often reversion happens

Mutation and reversion to a paralytic form is a rare but known risk of every live, weakened oral polio vaccine. Extensive use of nOPV2 since March 2021 suggests it happens less often than with mOPV2. A preliminary estimate puts emergences after mOPV2 at about one per 10 million doses, and after nOPV2 at about one per 100 million — roughly ten times lower.

Preventing emergences

As with every vaccine-derived outbreak, nOPV2-derived outbreaks are more likely when vaccination campaigns fail to reach high coverage among people with persistently low polio immunity. To stop type 2 outbreaks and prevent new emergences, the authors conclude, campaigns must be prompt and reach every targeted child.

Sources

• Elizabeth Davlantes, Jaume Jorba, Elizabeth Henderson, Kelley Bullard, Mark A. Deka, Anfumbom Kfutwah, Javier Martin, Maël Bessaud, Lester M. Shulman, Kaija Hawes, Ousmane M. Diop, Ananda S. Bandyopadhyay, Simona Zipursky and Cara C. Burns, "Notes from the Field: Circulating Vaccine-Derived Poliovirus Type 2 Emergences Linked to Novel Oral Poliovirus Vaccine Type 2 Use — Six African Countries, 2021–2023," Morbidity and Mortality Weekly Report, CDC: https://www.cdc.gov/mmwr/volumes/72/wr/mm7238a4.htm
• The source says the three possibly ongoing emergences were all from DRC, although one of the two that spread within their home country arose in the Central African Republic; this page does not repeat that attribution.