In the remote Chinese province of Guizhou, polio infections occurred in 2004, which researchers in The Journal of Infectious Diseases (2006;194:545-51) now say can be traced back to a mutated virus from the oral vaccine. Such infections are possible in all countries where vaccination coverage is low, an editorial said on the study. The oral polio vaccine consists of attenuated live virus that infects a child who receives the vaccine. Many children still shed the virus years later and can infect other children who have not been vaccinated with it. But as it turns out, it is sometimes possible for the virus to mutate back to the so-called ‘wild type’, allowing children infected with it to contract polio. Wild-type backmutation is quite possible, write Walter Dowdle and Olen Kew in an editorial (J Infect Dis 2006;194:539-41), because polio virus has a markedly high propensity to mutate. About 1 of his genes changes within a year. In addition, viruses can arise again that cause infantile paralysis. According to the authors, there are now 5 known polio outbreaks caused by vaccines. Except in China, these took place in Haiti, Egypt, the Philippines and Madagascar. What is special about the outbreak in China is that the country has had an ambitious vaccination program since the last major epidemic (in 1989-1991, with more than 10,000 deaths). In fact, more than 90 of all children should now be vaccinated. But it is likely that in Guizhou, one of the poorest regions in China, vaccinations have not been carried out consistently. Recent government data also shows this: Only 72 of the children there received the three doses of the vaccine. Between May and July 2004, 3 children contracted polio in 2 villages. The mutation rate of the virus suggested that it came from the vaccine that had been used in the villages less than a year earlier. The children were apparently not vaccinated at the time. Also 3 non-vaccinated contact persons of the children became ill. At the request of the government, all reported paralysis in the province were then examined and an infection with the same virus was found. Very soon afterwards, another vaccination program was started. Now more than 90 of the children under the age of 5 have been vaccinated.