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) announced on Thursday.
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 4-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, according to a press release from the World Health Organization (WHO).
GPEI said they will be supporting local authorities in the two nations “and neighboring countries to conduct a thorough risk assessment and plan vaccination responses to reduce the risk of further transmission, as per outbreak response protocols.”
These are the first instances of cVDPV2 linked with the nOPV2 since the vaccine became available in March 2021, GPEI noted. “While detection of these outbreaks is a tragedy for the families and communities affected, it is not unexpected with wider use of the vaccine.”
“Throughout the vaccine’s extensive field use, the strains in DRC and Burundi are the only two cVDPV2 emergences detected that have been linked with nOPV2,” the organization added.
Since March 2021, a total of 600 million doses of nOPV2 have been administered across 28 countries. GPEI estimated that 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 U.S., the injectable vaccine developed by Jonas Salk, MD, which contains inactivated poliovirus, has been used exclusively since 2000 due to the potential risk of vaccine-derived polio from products containing live virus.
According to the WHO, clinical trial data have confirmed the safety of nOPV2, and the non-inferior immunogenicity compared with mOPV2, adding that they are comparable in terms of viral fecal shedding parameters, with nOPV2 being more genetically stable than mOPV2.
Oral polio vaccination (OPV) comes with a risk, Vincent Racaniello, PhD, a virologist at Columbia University in New York City, told MedPage Today last year following the first U.S. case of polio in nearly a decade, which stemmed from vaccine-derived poliovirus type 2 in an unvaccinated New York state 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 approximately 3 million cases, it presents enough of a risk that polio vaccination can never be eased while it is in use. This is a conundrum previously dubbed the “OPV paradox.”
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.
“Circulating poliovirus type 2 infection can occur when the weakened strain of the virus contained in the oral polio vaccine circulates among under-immunized populations for long periods,” the WHO said in their press release.
Burundi has not seen cVDPV2 in more than three decades, the WHO noted.
“Polio is highly infectious and timely action is critical in protecting children through effective vaccination,” said Matshidiso Moeti, MBBS, the WHO Regional Director for Africa. “We are supporting the national efforts to ramp up polio vaccination to ensure that no child is missed and faces no risk of polio’s debilitating impact.”
Burundi health authorities are working with the WHO to continue sample collection to increase the ability to detect any poliovirus that may be circulating.
The Burundian government has declared the detection of the virus a national public health emergency, and plans to implement a vaccination campaign in the coming weeks for all eligible children ages 0-7 years.