without alarmism, experts warn

What have we discovered?

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

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

Poliomyelitis is a highly infectious viral disease that mainly affects children under 5 years of age. The virus, called poliovirus, is spread from person to person primarily by the fecal-oral route. Less frequently, it can be transported by ordinary means, 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 polio. Since then, cases of a wild virus have dropped by more than 99%, according to the WHO, largely thanks to vaccination.

What polio vaccines?

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

The injectable inactivated polio vaccine (IPV) requires multiple injections and regular boosters. Its cost has long limited its distribution to developed countries.

The less expensive oral polio vaccine (OPV) can be given without an injection. Quickly confers good general immunity and local immunity in the gut; It is also very cheap.

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

However, like wild poliovirus, the poliovirus in the vaccine can be passed 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 since 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 evaluates the probability of a case for about three million doses.

How to explain traces of polio in sewage?

According to UK authorities, the most likely scenario is that a newly vaccinated person 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 the eradication of polio is not yet complete worldwide,” David Heymann, professor of infectious disease epidemiology at the School of Hygiene and Tropical Medicine in New York, told Science Media Center (SMC). London.

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.

Therefore, the researchers consider it unlikely that this discovery poses a real risk in terms of public health.

But according to the WHO, “any form of poliovirus, wherever it is found, poses a threat to children around the world.”

What lessons can we draw from this?

For many experts, this discovery encourages the world population to be vaccinated to definitively eradicate the virus.

“This reminds us all how vital the global use of polio vaccines is,” said Beate Kampmann, director of the Center for Immunization at the London School of Hygiene and Tropical Medicine.

Because, he recalled, “the multiplication of vaccine-derived polioviruses can only occur if there is little immunity to polio 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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