For the first time, cases of paralytic polio have been linked to the novel oral polio vaccine type 2 (nOPV2), with multiple cases detected in Burundi and the Democratic Republic of Congo (DRC), the Global Polio Eradication Initiative (GPEI) said last week.
Circulating vaccine-derived poliovirus type 2 (cVDPV2) was found in stool samples from seven children with acute flaccid paralysis in the DRC and Burundi. Cases were confirmed in an unvaccinated four-year-old boy living in western Burundi and two other children who were in contact with the boy, as well as in five wastewater samples collected in Burundi, said the World Health Organisation (WHO).
GPEI will be supporting local authorities in both nations “and neighbouring countries to conduct a thorough risk assessment and plan vaccination responses”, reports Medpage Today.
These are the first cases of cVDPV2 linked with the nOPV2 since the vaccine became available in March 2021, GPEI said, adding that although these outbreaks were tragic, “it is not unexpected with wider use of the vaccine”.
“The strains in DRC and Burundi are the only two cVDPV2 emergences detected that have been linked with nOPV2,” it said.
Since March 2021, 600m doses of nOPV2 have been administered across 28 countries. GPEI estimated there would have been 30 to 40 new cVDPV2 emergences by now if the monovalent oral polio vaccine type 2 (mOPV2) had been used on the same scale as nOPV2.
In the US, the injectable vaccine developed by Dr Jonas Salk, which contains inactivated poliovirus, has been used exclusively since 2000 due to the potential risk of vaccine-derived polio from products containing live virus.
The WHO says clinical trial data have confirmed the safety of nOPV2, and the non-inferior immunogenicity compared with mOPV2, and that they are comparable in viral faecal shedding parameters, with nOPV2 being more genetically stable than mOPV2.
However, oral polio vaccination (OPV) comes with a risk, Vincent Racaniello, PhD, a virologist at Columbia University in New York City, said last year after the first US case of polio in nearly 10 years, stemming from vaccine-derived poliovirus type 2 in an unvaccinated New York resident.
"As long as we keep using oral polio vaccine in some countries, OPV-derived viruses will continue to circulate and pose a threat to any unvaccinated people," Racaniello said at that time.
Although the nOPV2 contains a live, attenuated poliovirus designed to live harmlessly in a person’s intestinal tract, it can mutate and revert to a version of the virus that can cause poliomyelitis in individuals who are not vaccinated.
Although this mutation is thought to occur as rarely as once in 3m cases, it presents enough of a risk that polio vaccination can never be eased while it is in use.
The nOPV2 is currently being monitored under the WHO Emergency Use Listing Procedure, and although these are the first cases associated with this vaccine, the WHO recorded more than 400 cases of cVDPV2 in 14 countries last year.
Burundi has not seen cVDPV2 in more than three decades, the WHO noted.
The Burundian Government has declared the detection of the virus a national public health emergency, and plans to implement a vaccination campaign for all eligible children aged under seven.
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