By Debora MacKenzie Live, weakened virus is used in some forms of the polio vaccinePIUS UTOMI EKPEI/AFP/Getty Images Nigeria has officially wiped out wild polio. It is three years since it had a case caused by the natural polio virus, a heartening milestone for a country that nearly derailed the global drive to eradicate the disease after some regions banned vaccination in 2003. But Faisal Shuaib, head of the country’s public health agency, called only for “cautious euphoria”. Nigeria has not wiped out polio. As first revealed by New Scientist in 2000, the live, weakened virus used in the oral polio vaccine responsible for this week’s victory is circulating and mutating back to its paralysing form. It has caused 15 cases in Nigeria so far this year. There are ways to stop this from happening, but they haven’t been rolled out fast enough, says Michel Zaffran, head of polio at the World Health Organization. Advertisement Meanwhile there have been three times more cases of wild polio virus this year in Pakistan and Afghanistan, which are now the only countries where it still circulates, than at this time last year, due to a lull in vaccination after a change of government in Pakistan, and a ban on it by the Afghan Taliban. Mutating virus Pakistan is now back on track, says Zaffran. But “we are in a very critical and dangerous situation,” he adds. Polio could roar back worse than ever if these resurgences are not contained. The drive to eradicate polio was based on a cheap, effective oral vaccine containing three strains of live, weakened polio virus. The Type 2 strain replicated faster than the others, provoking the most immunity, and as a result, wild Type 2 polio has been eradicated since 1999. But the Type 2 vaccine virus also tended to survive and circulate, sometimes reverting to the disease-causing form. So in 2016, the whole world shifted to a live vaccine containing only Types 1 and 3. Immunity to those improved, and cases fell. At the same time, children were supposed to get an injected vaccine containing killed polio viruses, all three strains, making them immune to any vaccine-derived virus still circulating. In this way, India eradicated all polio in 2014. But too few children in poorer nations, including many African countries, get routine vaccination, so “there have been more outbreaks of Type 2 vaccine-derived virus than we expected,” says Zaffran. The only way to stop such outbreaks spreading is to give people a live, oral vaccine containing only Type 2. International outbreaks This is because, while the injected vaccine will stop you getting infected, once you are infected only the live vaccine will create the right immunity in your gut to stop you spreading the virus. And in an outbreak, 95 per cent or more of people infected don’t get sick, but spread the virus, so many must be vaccinated. But the live, Type 2 vaccine also spawns yet more potentially dangerous vaccine-derived virus. And we could run out – only three companies make it. “We have enough to cope now, but there could be a crisis if the outbreaks don’t improve,” says Zaffran. Yet there are ever more people susceptible to Type 2 polio, as children are born after the change in the oral vaccine, natural polio no longer circulates and immunises people, and too few get the injected vaccine. All this is why the WHO has called the polio situation a Public Health Emergency of International Concern since 2014. Outbreaks have spread internationally: Nigeria sent neighbouring Benin its first case of vaccine-derived virus in July, and Iran detected wild polio from Pakistan in sewage, near the border, earlier this year. It didn’t get far: most Iranians are vaccinated. Zaffran worries about Iraq or Yemen with their demolished health systems. Yet the WHO lacks two-thirds of the $5.2 billion it estimated is needed to finish polio. It plans to ask for funds in November. More on these topics: diseases infections vaccines polio
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