On 17 March 2023, the World Health Organization (WHO) announced that health officials in Burundi and the Democratic Republic of Congo (DRC) had detected cases of vaccine‑derived poliovirus. The Burundian government declared the detection a national public‑health emergency after three cases were confirmed. In an interview with The Conversation Africa, virologist Oyewale Tomori explained the landscape of polio viruses, their mutations, and the current vaccine response.
There are two types of polioviruses that can cause paralysis in humans: wild poliovirus and vaccine‑derived poliovirus. Wild poliovirus, the form most people recognize, has three serotypes (1, 2 and 3). Global immunisation campaigns using the oral polio vaccine (OPV) since 1988 have reduced wild‑polio cases by 99.9 %. Today, only wild poliovirus type 1 remains in circulation, confined to Pakistan and Afghanistan. A second form, circulating vaccine‑derived poliovirus (cVDPV), can also spread within communities. Although rare, cVDPV cases have risen in recent years because of low immunisation rates. Over the past two decades, mutated forms of the vaccine virus have emerged in vaccinated individuals; these are the strains detected in the recent Burundi and DRC outbreaks. While the mutations differ from wild poliovirus, they can still cause paralysis, prompting the development of a vaccine specifically targeting the mutated variety.
Two polio vaccines are used worldwide: the oral polio vaccine (OPV) and the inactivated polio vaccine (IPV). OPV, which contains an attenuated (weakened) virus, has been instrumental in driving wild poliovirus to the brink of eradication. It is safe and induces strong gut immunity, the site where poliovirus replicates, unlike IPV. The attenuated virus can be excreted in stool, which can be advantageous in areas with poor sanitation because the vaccine virus can spread from person to person and indirectly protect the community—provided it does not have time to mutate. In settings with low immunisation coverage, however, the vaccine virus may circulate for 12–18 months, during which it can acquire mutations that restore neurovirulence, resulting in vaccine‑derived poliovirus. When such mutated virus spreads widely, it is termed circulating vaccine‑derived poliovirus (cVDPV). More than 10 billion doses of OPV have been administered to nearly three billion children since 2000, and about 2 500 cases of paralysis from cVDPV have been recorded. These cases do not signal a resurgence of wild poliovirus.
Since the first report of vaccine‑derived poliovirus in Hispaniola in 2000, three types of cVDPV have been identified in over 50 countries across every region except Antarctica: cVDPV type 1, type 2 and type 3. Between 2000 and mid‑March 2023, 213 of 399 reported cVDPV‑1 isolates (53.4 %) were linked to human paralysis, with the remainder found in healthy contacts or environmental samples. cVDPV‑2 accounted for 2 407 paralysis cases, representing 53.4 % of 4 507 isolates, while the remaining isolates were recovered from contacts and sewage. Only 53 cVDPV‑3 isolates were reported between 2020 and 15 March 2023; a single case caused paralysis, and 52 isolates came from environmental samples.
To curb the spread of cVDPV, the Global Polio Eradication Initiative and partners introduced a novel oral polio vaccine type 2 (nOPV2) in March 2021. nOPV2 is a genetically stabilised version of the monovalent OPV2 (mOPV2) currently used to respond to cVDPV‑2 outbreaks. Clinical trials have shown that nOPV2 provides protection comparable to the older vaccine while being less likely to revert to a neurovirulent form, thereby reducing the risk of new mutations entering communities. By March 2023, close to 600 million doses had been administered across 28 countries, interrupting cVDPV‑2 transmission after two immunisation rounds in 14 of 17 countries that used the vaccine. In Tajikistan, two large campaigns followed by a smaller one halted transmission. However, local transmission persists in northern Nigeria despite several nOPV2 campaigns, and the recent cVDPV‑2 detections in Burundi and the DRC are the first such outbreaks since nOPV2 rollout began.
The outbreaks underscore that vaccination alone is insufficient to stop local transmission; reaching vulnerable, under‑immunised children before the virus mutates is equally crucial. The new vaccine is a supplementary tool that enhances immunisation and protection against cVDPV outbreaks, but countries must ensure comprehensive, full immunisation of every child to prevent vaccine‑preventable diseases. There is no shortcut to achieving complete coverage.
— Oyewale Tomori, Fellow, Nigerian Academy of Science
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