What is vaccine-derived polio?

By S.C.

AS POLIO ERADICATION slides through the final stretch of a 30-year global campaign to wipe it out, a new worry has emerged: vaccine-derived cases of the crippling disease. Such cases are extremely rare, but are attracting more notice as those caused by the wild virus itself have dwindled. In 2017 cases caused by vaccine-derived viruses overtook, for the first time, those caused by the wild version. The tally for 2018 shows a dramatic swing: 98 cases of vaccine-derived polio; 29 cases of the wild version. What is vaccine-derived polio?

Polio vaccines come in two forms. The injectable version, used in rich countries, contains dead viruses and creates antibodies in the blood. Someone vaccinated with it who ingests the wild virus (say, by drinking contaminated water) is protected from the disease. But, for several weeks afterwards, the wild virus in that person’s gut can be passed on to people who are not immune. The oral vaccine, by contrast, contains weakened live virus. Because the antibodies it creates take up residence in the gut, they battle there with any wild virus a vaccinated person might ingest, making further transmission less likely. The oral vaccine is therefore a better option in places, often poor countries, where wild polio viruses are common and vaccination rates are low. Moreover, someone vaccinated with the oral vaccine excretes the weakened form of the virus for a couple of weeks. Anyone who comes into contact with this excreted virus also gains immunity, and can pass it on to others who are not immune. This sort of passive vaccination is a boon—but only up to a point. As the weakened virus from the vaccine jumps from one unvaccinated person to another, the chances increase that something will go wrong. Along the way, the virus mutates and, after a year or so, can turn into a paralysing form that resembles the wild virus.

Of the three strains in which poliovirus exists, type 2 is most adept at this trick. It causes more than 90% of paralytic polio cases from mutated oral-vaccine strains. So when, in 2015, the wild type 2 poliovirus was declared eradicated, it made sense to stop vaccinating people against it. In 2016, in a co-ordinated switch that took place over the course of two weeks, 155 countries replaced their stocks of oral polio vaccine containing all three strains with a version that does not include the type 2 strain. To protect people from any type 2 vaccine-derived virus still circulating, the injectable vaccine was added to routine immunisation schedules in these countries. But gaps in vaccination coverage have prevented such type 2 mutations from dying out. In 2018 they caused cases of paralytic polio in the Democratic Republic of Congo, Nigeria, Niger and Somalia. Genomic analysis of the strains involved showed that they had crossed borders, which is rare for vaccine-derived strains, and that some had circulated undetected for as long as four years. Health officials worry that the outbreaks may spread to neighbouring countries.

That is a setback for Africa. The last person on the continent paralysed by the wild polio virus was a Nigerian child who contracted the disease in 2016, so Africa has probably already eradicated the wild virus. This leaves Afghanistan and Pakistan as its last two strongholds. But the outbreaks of vaccine-derived cases are a sign that polio’s grand finale may be more drawn out than even pessimists expected. When wild polio virus disappears, the oral vaccine will be replaced with the injectable vaccine. How long such jabs will be needed to guard against the remnants of vaccine-derived polio is anybody’s guess.