IS polio back?

The news that polio is spreading anywhere in the UK for the first time in nearly 40 years is understandably cause for a robust public health response, but so far there no cause for alarm.

Back in 1947, when Britain was hit by its worst ever outbreak of poliomyelitis, there were 1,434 confirmed cases in Scotland and 131 deaths - mostly in children under five.

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In 1950s America, polio was said to be "second only to the atomic bomb" as the thing citizens dreaded the most, with summer outbreaks leaving hundreds of children paralysed or dead every year, and many more disabled for life.

Thanks to a highly effective vaccine, the threat has been largely eradicated - although the case of an unvaccinated 20-year-old New York man paralysed by an infection caused by the same strain which appears to be circulating now in London is a reminder never to be complacent when it comes to potentially deadly viruses.

Ironically though, the source of the current outbreak is a form of the vaccine. The oral polio vaccine contains a weakened, live poliovirus and is used in countries where polio is still present because it stops the spread of the virus by inducing immunity in the gut, where it replicates.

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Individuals vaccinated in this way can excrete the virus in their faeces (hence why polio is monitored through sewage surveillance). It is likely that a visitor to London introduced the virus in this way.

Usually these cases are self-contained; but not this time. Genetic mutations shared some of the samples (and the New York case) show that it is transmitting, albeit in small numbers .

The race is now on to ramp up vaccine coverage in London, targeting the communities where uptake in children is lowest, in order to build a wall of immunity to block off the virus and contain it.