Bengaluru: After smallpox, the second disease in humans that has come close to eradication is the deformity- and paralysis-inducing polio, which tends to mainly affect children.
Earlier this year, 47 African nations were certified to be free of wild polio, or the form of poliovirus that is found naturally.
However, another form of polio still persists in many places — driving up the risk of an outbreak as vaccination drives withdraw — the rare form of the disease that spreads from oral vaccination itself in under-immunised populations.
Vaccine-derived polio still poses a threat in many nations in Africa and Asia, having spread naturally from the weakened polio virus that is used in vaccination.
To prevent such outbreaks caused by vaccines, the World Health Organization (WHO) is set to approve kind of polio vaccine, developed through a consortium of global partners, for emergency authorised use before the end of this year, said a report in Nature.
This novel OPV type 2 vaccine will mark the first time an unlicensed vaccine passes through the WHO’s emergency listing process.
What is vaccine-driven polio?
The WHO’S ambitious polio eradication programme has conducted thousands of vaccination drives in multiple parts of the world since 1988, to great success. Polio is well on its way to eradication in humans, after killing over 500,000 individuals annually in the 1940s and 1950s at its peak.
The number of infections globally has dropped from over 350,000 in 1988 to just 33 in 2018, although the pandemic now threatens the eradication drive, setting it back by decades.
Wild polio is a threat only in two countries today: Pockets of Afghanistan and parts of Pakistan, both of which are disproportionately affected by medical disinformation about vaccines as well as hostile mountain terrain that makes vaccination drives difficult.
Today, cases of circulating vaccine-derived poliovirus (cVDPV) are found in 19 African countries, but also in the Philippines, Malaysia, Yemen, China as well as Afghanistan and Pakistan. In 2020 alone, the number of individuals infected with vaccine-derived polio has risen to 531, compared to 366 last year.
Vaccine-derived polio is typically caused by one of the three strains of the poliovirus, the type 2. Type 2 and 3 are also considered to be eradicated in the wild entirely as of 2019, and type 1 is what still circulates in Afghanistan and Pakistan.
However, tackling the existing outbreaks, even of vaccine-driven type 2 infections is done through precisely the same vaccines, which further increase the risk of an outbreak. This is caused by the extremely efficient oral polio vaccine (OPV), which was administered last in 2016.
The OPV consists of a live virus, which starts to immunise the intestines, where the poliovirus first infects the body. It has been safely administered to over 3 billion children, and is the primary reason for the world’s growing herd immunity to polio.
Just like the natural virus, the weakened virus can pass from child-to-child and confer immunity even to unvaccinated ones.
In very rare cases, and in under-immunised communities, the weakened virus being shed by vaccinated children can circulate in the population. As it spreads from child to child, it mutates. Over long periods of time, when large numbers of children are unvaccinated, the weakened virus mutates enough to eventually revert to its more virulent, paralysis-causing form. This poses a risk to unvaccinated children in these communities, sometimes leading to outbreaks.
This is what happened in multiple parts of the world, including in India, where the last wild type 2 poliovirus was seen in 1999 but there were vaccine-derived type 2 outbreaks or cases up until 2013.
The injected form of the poliovirus vaccine is safer but is less effective, does not immunise the intestine, and is considered to have high risk during manufacturing. Furthermore, these vaccines will not provide immunity to vaccine-derived strains already circulating.
“IPV protects against all forms of polio just as OPV does, but it has a different method of action: it only protects the individual, but does little to prevent onward spread of the virus,” clarified a spokesperson for WHO’s Global Polio Eradication Initiative. “OPV has a unique profile to interrupt person-to-person spread, hence why it is the primary tool in polio eradication.”
Phasing out oral vaccines also can leave communities vulnerable to existing circulating vaccine-derived strains, posing a dilemma.
The new vaccine
Like the OPV, the new vaccine — novel OPV type 2 — also uses a form of a live virus, but it has been genetically modified to prevent it from becoming virulent.
Researchers at National Institute for Biological Standards and Control, UK, and the US Centers for Disease Control and Prevention, started to work on understanding the parts of the viral RNA which were mutating in the vaccines to revert the virus to a more virulent state.
The team edited these parts of the RNA at crucial points making it harder for the virus to undo the edits and become more virulent.
The scientists additionally modified the virus to not recombine with other viruses found in the human intestines. They also modified its RNA so as to slow down its speed of evolution and mutation.
This new vaccine was still powerful enough to evoke an immune response through vaccination but weak enough to not mutate into a virulent form of the virus. The vaccine went into phase 1 safety trials in 2015, funded by the Bill and Melinda Gates Foundation. Two phase 2 trials have been completed but results are as yet unpublished.
Much larger scale trials are still needed, but the risk of countering smaller outbreaks with more of the existing vaccines is increasing the risk of losing control over polio, say experts. Although the theoretical risk of the new vaccine not causing polio again cannot definitively be zero without bigger trials, emergency authorisation is the need of the hour, they say.
Emergency use listing was a procedure that was put in place during the 2014-16 Ebola outbreak in West Africa and will also be used for emergency Covid-19 vaccines.
This report has been updated to include a WHO spokesperson comment.