Nigeria’s polio-free status at risk as new strain spreads

After achieving a monumental milestone of eradicating the Wild Poliovirus (WPV) in 2020, Nigeria’s hard-won status is now under threat as a new strain of the virus—Circulating Vaccine-Derived Poliovirus Type 2 (cVDPV2)—continues to spread in the country.

This persistent spread, driven largely driven by low immunisation rates and failure to achieve herd immunity against the new strain especially in Northern Nigeria, has raised concerns among public health experts of a potential outbreak which would be a setback for the polio-free milestone that has been achieved.

The National Primary Health Care Development Agency (NPHCDA) reported last month that the spread of the mutant polio virus is intensifying. Data from the agency and  the World Health Organization(WHO) show that since 2021, over 1,270 cases of this mutant strain have been recorded, excluding 2022 figures, with 73 cases reported from 46 local councils across 14 Northern states as at September,  2024.

“Nigeria is witnessing intense transmission of the circulating variant poliovirus type 2, with 70 cVPV2 from 46 local councils across 14 Northern states. This is an indication of continuous transmission of the virus due to low routine immunisation, and refusal of vaccines during polio campaigns”, Muyi Aina, executive director, NPHCDA, told northern religious leaders at a meeting organised late September.

Adaobi Onyechi, a public health expert, explained that Type 2 polio is genetically unstable and mutates rapidly, posing a significant threat to unvaccinated children. She  noted that the vaccine-derived strain has mutated extensively and poses a significant risk of causing paralysis akin to that of wild poliovirus.

According to Secience Direct, the Sabin-strain (live-attenuated) virus in oral poliovirus vaccine (OPV) can revert to circulating vaccine-derived poliovirus in under-vaccinated communities, regain neurovirulence and transmissibility, and cause paralysis outbreaks.  The fecal-oral route transmission of this virus culminates in Acute Flaccid Paralysis (AFP), a serious manifestation reminiscent of the Wild Polio era.

Between January 2016 and December 2020, the cVDPV2 caused 1,596 acute flaccid paralysis cases across four World Health Organization regions with 962 (60.3%) cases occurred in African Region.

In his latest visit to Nigeria in September, Bill Gates expressed concern over the persistent transmission of this variant.

Read also: Polio: WHO, Borno partner on immunisation of 2.5m million children

“This circulating variant, poliovirus type  is equally bad as the wild poliovirus. It  can paralyze or even kill children, and we still have work to do to get rid of this,” Gates said.

Gates also told the 144th edition of National Economic Council Meeting (NEC) chaired by Vice President Kashim Shettima that the country needs to urgently address the challenge.

The NPHCDA, in response to a BusinessDay inquiry, attributed the current rise of polio variant to the rainy season, which is  the highest transmission period for the virus (rainy session). The agency also said low population immunity in affected locations due to poor routine immunisation uptake and lack of herd immunity in affected locations are among leading factors driving the spread.

The agency, however, assured that Nigeria’s polio-free status remains intact, as it implements targeted responses such as immediate outbreak response (OBR), supplemental immunisation activities (SIAs), and between-round efforts to reach children missed in earlier campaigns.

Nonetheless, experts stress that these efforts are not enough and need to be more robust and reach remote communities, especially in conflict zones.

A 2024 report entitled, “Modeling the spread of circulating vaccine-derived poliovirus type 2 outbreaks and interventions: A case study of Nigeria,” published by Science Direct, reported that the current essential immunisation coverage levels are not sufficient to stop transmission of the virus.

The report found that earlier and more rounds of supplemental immunisation are needed, especially in regions with low immunisation coverage.

Nigeria still has approximately 2.3 million children who have never been vaccinated. Chika offor, the CEO of Vaccine Network for Disease Control (VNDC), in an interview with BusinessDay, said addressing zero-dose requires a more robust multi- stakeholder approach.

“We have a huge zero dose vaccine gap, covid-19 stalled a lot of this. All hands must be on deck. The financing is also very important because the vaccines must be available,” she said.

The report further highlighted that the cVDPV2 transmission persists because of insufficient and delayed pre- and post-switch vaccination campaigns caused by insurgency, which has deprived many children of vaccination.

Though Nigeria in March 2021 rolled out a novel type of OPV2 (nOPV2) with a lower risk of virus reversion outbreak response SIAs, the report found that effectiveness (quality) of some of these SIAs was compromised.

According to the report,  the incompletion of outbreak response plans led to households/communities being overlooked in house-to-house campaigns. Vaccines failed to reach children in the most critical transmission areas especially due to insecurity. Consequently, despite the adoption of nOPV2, existing outbreaks persisted through multiple SIA rounds.

The report found that using monovalent OPV2 (mOPV2) provides a chance for effective vaccinations with markedly reduced risk of seeding new cVDPV2 emergence. However, as evidenced by persistent outbreaks in 2021 and 2022 to date , it notes that safer vaccine does not compensate for low-quality outbreak response vaccination campaigns which fail to quickly immunize all target children.

The report recommended the need to keep identifying under-vaccinated population areas and conduct effective vaccination campaigns to boost population immunity and to prevent possibly prolonged transmission in these areas.

Global bodies like the UNICEF and WHO said the resurgence calls for concerted efforts to address root causes such as poor sanitation system, inability to achieve herd immunity of the cVDPV2 vaccine. They also emphasized the need  address the root causes of declining immunization rates, such as prevailing myths and  misconceptions to close the vaccine gap.