I N 1988 A world emboldened by the eradication of smallpox set its cross-hairs on polio. The aim was to enter the new millennium without this crippling virus. But the battle drags on. On November 30th an emergency committee on the global spread of polio, appointed by the World Health Organisation ( WHO ), delivered its latest verdict. Eliminating polio has become what Michel Zaffran, director of polio eradication at the WHO , calls a “dual emergency”.

The first is the stalled progress on wiping out wild polio viruses in their last two strongholds, Afghanistan and Pakistan. The world’s steady countdown to zero since 2012 has stopped, with cases this year already surpassing those in 2017 (see chart). As a consequence there are worries that polio may travel back to countries which have already eradicated it, like India.

The second emergency is the growing number of countries with cases of the disease that have been caused by a polio vaccine. These are still rare. But they 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.

Polio vaccines come in two forms. The injectable version, which rich countries use, 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 his 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 ingests, making further transmission less likely. The oral vaccine is thus a better option where wild polio viruses roam and vaccination rates are low—which has been the case in poor countries. The oral vaccine has another benefit. Someone vaccinated with it 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 further, to others who are not immune. In places with poor sanitation, 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 polio virus 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.

Yet last year type 2 viruses derived from the oral vaccine caused cases of polio in Syria and the Democratic Republic of Congo ( DRC ). This year cases emerged in Nigeria, Niger, Somalia and the DRC —a sign that gaps in vaccination coverage are widespread. Genomic analysis of the strains involved showed that they had crossed borders (rare for vaccine-derived strains) and that some had circulated undetected for as long as four years. Health officials worry that the outbreaks in Somalia, in particular, 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. The outbreaks are also 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 remnants of vaccine-derived polio is anybody’s guess.