Why has polio returned to London and New York?

POLIOMYELITIS (OR POLIO, for short) terrorised America and Europe until the 1960s. It paralysed more than 15,000 people a year in America and 8,000 in Britain, mostly children. There is no treatment for polio and 5-10% of paralytic cases are fatal. The invention of a vaccine set the world on a path to eradication. The Americas region was declared free of the wild poliovirus in 1994. Europe followed in 2002 and Africa in 2020. But in June a 20-year old man was paralysed by polio in a suburb of New York City. It was America’s first confirmed case of local transmission since 1979. In recent months poliovirus has turned up in wastewater samples in London and New York in quantities that suggest the virus is circulating there. Why has polio returned?

Today, the wild poliovirus is endemic only in pockets of Afghanistan and Pakistan. The type detected in London and New York is a vaccine-derived strain, a rare mutation originating from the oral polio vaccine. Unlike the jab used in Western countries, which is based on an inactivated virus, the oral vaccine contains a weakened strain of the live virus. It is still used in poor countries for several reasons. First, it is cheaper and easier to administer. Second, a person who takes the oral vaccine can transmit immunity through “passive inoculation”: they shed the weakened virus in their stools for a few weeks after vaccination. If an unvaccinated person ingests particles of the virus (which is common in places with poor sanitation) that person becomes inoculated too.