Traces of polio in London: what we know

The discovery of traces of a form of poliomyelitis in London presents little immediate risk. And, even if they come from a vaccine-derived strain, it’s more of an incentive to speed up vaccination, experts say.

What have we discovered?

Traces of a form of the poliomyelitis virus have been found in samples of wastewater from a London sewage treatment plant, the World Health Organization and British authorities announced on Wednesday.

This is not a strain of virus in the wild, but a “derived” version that is used to vaccinate against the disease.

Polio is a highly infectious viral disease that largely affects children under the age of 5. The virus, called poliovirus, is transmitted from person to person mainly by the faecal-oral route. Less frequently, it can be conveyed by an ordinary medium, for example contaminated water or food.

It multiplies in the intestine, from where it can invade the nervous system and cause paralysis.

In 1988, the World Health Organization passed a resolution calling for the global eradication of poliomyelitis. Since then, cases due to a wild virus have fallen by more than 99%, according to the WHO, largely thanks to vaccination.

Which vaccines against polio?

If polio is an incurable disease, it is indeed possible to prevent it through vaccination. The disease was widespread all over the world, until the discovery of a vaccine in the 1950s. There are currently two types.

The injectable inactivated poliomyelitis vaccine (IPV) requires several injections and regular reminders. Its cost has long limited its distribution to developed countries.

The less expensive oral polio vaccine (OPV) can be given without injection. It rapidly confers good general immunity and local immunity in the intestine; it is also very affordable.

Each drop of vaccine contains the attenuated poliovirus. Thus vaccinated children can develop immunity against poliomyelitis.

However, like the wild poliovirus, the vaccine poliovirus can be transmitted from one child to another. In this way, other children can be protected even if they are not vaccinated.

But when many children are not vaccinated and immunity levels are very low, the vaccine virus can continue to spread.

And as it infects new unvaccinated children, it may undergo slight genetic changes. This is called vaccine-derived poliovirus (cVDPV).

In very rare cases, they cause paralysis, explains the WHO, which assesses the probability of one case for about three million doses.

How to explain traces of polio in wastewater?

According to UK authorities, the most likely scenario is that a recently vaccinated individual entered the UK before February from a country where oral polio vaccine (OPV) has been used in vaccination campaigns.

“The presence of this virus reminds us that polio eradication is not yet complete worldwide,” David Heymann, professor of infectious disease epidemiology at the London School of Hygiene & Tropical Medicine, told Science Media Center (SMC).

What is the threat?

“It is important to note that the virus was isolated only from environmental samples – no associated cases of paralysis were detected,” the WHO said on Wednesday.

The researchers therefore consider it unlikely that this discovery will lead to a real risk in terms of public health.

But according to the WHO, “any form of poliovirus, wherever found, poses a threat to children everywhere.”

What lessons can we draw from this?

For many experts, this discovery encourages to vaccinate the world population in order to eradicate the virus definitively.

“This reminds us all of how vital the global use of polio vaccines is,” said Beate Kampmann, director of the London School of Hygiene and Tropical Medicine’s Vaccination Centre.

Because, she recalled, “multiplication of vaccine-derived poliomyelitis viruses can only occur if there is low immunity against poliomyelitis in a community”.

Other researchers have emphasized the importance of monitoring wastewater in this way.

This helps “detect the circulation of a variety of infections in the general population,” said Jonathan Ball, professor of molecular virology at the University of Nottingham at SMC.

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