Why a danger that we have almost forgotten is reviving

Polioviruses have been detected in New York State, London and Jerusalem in recent months - but many more countries could be affected. “Ultimately, polio can occur anywhere,” says Sabine Diedrich from the Robert Koch Institute (RKI) in Berlin. "You only find what you're looking for." In the US, authorities took notice after a young man in Rockland County, which borders New York, contracted polio - the first case of polio in the US in almost a decade.

According to the US health authority CDC, the pathogen could have been circulating in the region unnoticed for up to a year. "If you have a polio patient with paralysis, you know immediately that there is a bigger problem," explains polio expert Diedrich. Because only in about one in 200 cases does an infection lead to the irreversible paralysis typical of polio - and that only in unvaccinated people. Such a case can therefore mean hundreds of infected people without symptoms in the region.

The poliovirus is a so-called enterovirus that primarily infects the digestive tract. An unvaccinated person will develop a flu-like illness in about a quarter of cases. Meningitis develops in one to five out of every hundred infected people, and even more rarely the spinal cord is infected and paralysis occurs, which in isolated cases also affects the respiratory muscles and is fatal. Before the introduction of vaccinations, there were thousands of sick people and hundreds of deaths every year in Germany alone.

Polio is also called polio because the pathogen was once so widespread that contact with it usually occurred in childhood. Small children in particular were affected by poliotypical paralysis. "These are usually permanent damage for life," explains Diedrich. Individual limbs are affected, sometimes both arms and legs.

"So far, there is no therapy." In addition to acute paralysis, post-polio syndrome, which is also associated with paralysis, can occur decades after an infection. "In Germany alone there are still thousands of people who were infected before the vaccinations were introduced." The syndrome often means life in a wheelchair.

The worldwide vaccination campaigns initiated in 1988 have so far saved around 20 million people from paralysis and one and a half million from death, according to the World Health Organization (WHO). In the meantime, however, the vaccination rates are far too low in many places. The head of the New York health authority, Ashwin Vasan, recently called for vaccination: "The risk for New Yorkers is real, but protection is so simple - get vaccinated against polio." In the metropolis, 14 percent of children are between the ages of six months and five years not or not fully vaccinated against polio.

Also in London there was the urgent advice to be vaccinated. The vaccination rate is only about 50 percent in some of the affected parts of the city, says Diedrich. "This virus is very, very good at finding unvaccinated people," epidemiologist Walter Orenstein of Emory University in Atlanta recently warned in the journal Nature. In his opinion, the currently known cases are only "the tip of the iceberg".

In London, health authorities became aware in June when polioviruses were repeatedly found in sewage samples. In Israel, the pathogen was first detected in a four-year-old child in Jerusalem at the beginning of March. Subsequently, more cases and polioviruses were found in the sewage of several cities in the country.

How can that be, although polio has been considered almost eradicated for years? The pathogens detected in the three countries are not the wild type of the polio virus, but viruses that go back to the oral vaccination with weakened but living polio pathogens. They can be excreted by the vaccinated person for up to six weeks. Initially, infection via saliva and throat secretions is also possible, as Diedrich explains.

Both the vaccinated person and contact persons can - in very rare cases - contract what is known as vaccine polio, the symptoms of which cannot be distinguished from polio caused by wild viruses. A continuous multiplication of the vaccine virus carries the risk that the weakened pathogen changes and is able to infect the nervous system - with the poliotypical paralysis as a possible consequence.

The highly contagious virus is usually spread via contaminated hands as a so-called smear infection, in countries with insufficient hygiene standards also via contaminated water, as Diedrich says. It does not last long on surfaces such as a door handle, but this can also be a way of infection if there is a lack of hand hygiene, for example if an infected person has visited a public toilet immediately beforehand.

Israel has been using oral live vaccines (OPV) for years, but the US and UK have not. Inactivated vaccines (IPV) that contain no viable viruses and are injected have been in use there for a long time - as in Germany exclusively since 1998. The pathogens circulating in London and New York were probably first introduced by people who had received the oral vaccination, which is still widespread in their country.

But if there is a harmless alternative, why is the oral vaccination still in use at all? The main cause is a peculiarity of the inactivated polio vaccines: They prevent diseases very well, but not an infection and the transmission of the pathogen. As a result, the virus can spread widely unnoticed. This can be dangerous, especially in countries with low vaccination rates.

In Africa and Asia in particular, oral vaccination is still widely used, which also protects against infection and thus against widespread transmission. It is also cheaper than the inactivated vaccine and not only protects the vaccinated person as with the HIV vaccine, but can be transmitted to other people in the area - which means that they are also vaccinated. The very low risk of a vaccination polio case is accepted in favor of a large-scale immunization of the population.

The goal of making polio history after the eradication of smallpox in 1980 has so far not been achieved. Until recently, wild polio viruses circulated practically only in Pakistan and Afghanistan, with only a few cases in each. This year, however, for the first time since 2016, evidence was reported in African countries - which had only been declared polio-free in 2020.

Routine vaccinations such as those against polio were interrupted in many countries during the pandemic years, WHO expert Oliver Rosenbauer recently explained. "In some regions, children are now at higher risk from infections like polio," he said. "This also increases the risk of polio spreading internationally again."

In Germany, babies are vaccinated from the age of two months, and according to RKI expert Diedrich, the vaccination rate is around 90 percent nationwide. "That's not enough," she emphasizes. The rate is particularly low in Bavaria and Baden-Württemberg, for example. One problem in this country is that the collective memory has lost the terrible consequences polio had for countless children - and could have again. "This is by no means to be taken lightly."

There is now a new live vaccine with a virus that is less able to multiply and is not excreted for so long, says Diedrich. But the key to eliminating polio remains the eradication of the wild virus. It can only be achieved with global vaccination rates of around 95 percent. "Then the virus wouldn't have a chance." At the moment, the rates are often below 80 percent.

Experts are by no means surprised by the current outbreaks. As little attention was paid to it in wealthier countries: "In the last 20 years there have been many, many serious outbreaks in developing countries worldwide," emphasizes Oliver Rosenbauer from the Global Polio Eradication Initiative of the WHO in the journal "Nature". A plan and the political will are needed to be able to vaccinate children in regions such as war zones. Otherwise, "the disease will return worldwide".