By Debora MacKenzie Another victory for vaccinesGetty Images Humanity has destroyed another enemy. On 24 October the World Health Organization is expected to announce that wild polio virus Type 3 has been eradicated. Type 2 has been gone since 1999. That leaves only Type 1, in Pakistan and Afghanistan. But vaccine resistance there has led to 88 cases so far this year, after only 33 all last year. So while the disappearance of Type 3 is a welcome triumph, the WHO is still calling polio a Public Health Emergency of International Concern – and the emergency is now reaching a crunch point. “We face a very hard twelve months,” says Michel Zaffran, head of polio at WHO. Advertisement That isn’t only due to problems in Pakistan. As New Scientist revealed in 2000, the weakened, live virus in oral polio vaccine can persist and revert to the disease-causing form. The Type 2 vaccine virus replicates rapidly in people, provoking strong immunity which helped wipe out wild Type 2 polio while the other two types hung on. Unfortunately it also made the type 2 vaccine virus more likely to persist and revert to being able to cause paralysis. Since 2017 such reverted vaccine viruses have caused more cases of paralysis than actual wild polio virus. To solve the problem, in 2016 the entire world switched to a live vaccine with only Types 1 and 3. Immunity to those leapt, leading to this week’s victory over Type 3. Children born since then, however, can transmit outbreaks of Type 2 vaccine virus, as they haven’t had the live Type 2 vaccine. As time goes on and more are born, says Oliver Rosenbauer of the WHO, those outbreaks get more risky. Read more: Every country worldwide is now using the most effective polio vaccine And there have been more outbreaks than predicted in 2016: ten in Africa this year, including countries such as Zambia with low levels of immunity, and two worrying cases in China and the Philippines. The outbreaks can be stopped with live, type 2 vaccine, says Zaffran. The dilemma is that it seeds more reverted vaccine virus. Now there may be a way out: a novel Type 2 vaccine virus, genetically altered to be less likely to revert. In June virologists in Antwerp, Belgium reported that the vaccine was safe and successfully induced immunity in 30 people – who were isolated for a month in a purpose-built “poliopolis” that kept the virus from escaping. It is undergoing further trials, but is already being used by an Indonesian firm, Biofarma, to make live Type 2 vaccine. If all goes well, 100 million doses will be deployed in June next year. Then it will be a race to snuff out vaccine virus outbreaks faster than new ones start, while hopefully seeding no more. Yet now the polio programme doesn’t have the money it needs. It will ask donors next month. More on these topics: vaccines