The main vaccine being used to eradicate polio consists of a live, weakened form of the virus. If that happens after vaccination stops, unvaccinated children would be defenceless. Polio experts raised these concerns at a meeting in Geneva last month. Tjeerd Kimman, of the National Institute of Public Health and the Environment in the Netherlands, warned against stopping vaccinating once the disease had been eradicated. The polio virus, which is spread by food and water contaminated with faeces, does not cause serious symptoms in most people, but if it spreads to the nervous system it can be deadly.
Children are particularly vulnerable to the virus.
In 1988, when the WHO launched its global eradication campaign, there were some 35,000 cases of polio worldwide. But, ten years later, with 82 per cent of the world's children vaccinated, this number had dropped to just 3,200. OPV is cheaper than alternative vaccines, easier to administer and has the advantage of inducing immunity in the gut, where the virus first invades. Because it is 'live', OPV viruses also spread in faeces, immunising unvaccinated people who come into contact with vaccinated children. But OPV viruses differ from their nastier relatives by only a few mutations.