Polio can spread despite high vaccination rates

(CNN) — So far, there has only been one recent case of polio in the United States, which paralyzed a young adult in Rockland County, New York.

One case, and the evidence in sewage of further spread, may not seem significant, and Americans are highly vaccinated against polio, meaning most people are protected from paralysis. According to the latest data from the US Centers for Disease Control and Prevention (CDC), more than 92% of children in the United States have received polio vaccinations by age two. year old.

But there is a reason why a single case puts health authorities on alert. The unvaccinated and undervaccinated are at risk of severe disease, but the spread may not be obvious, in part due to a vaccine switch that occurred more than two decades ago.

Vaccinated people are not at risk if they contract poliovirus, the virus that causes poliomyelitis, which is spread from the human intestinal tract by the fecal-oral route: a person gets germs from feces on their hands, touches something, or gives the hand to another person, and that person puts their contaminated hands in their nose or mouth. Therefore, the youngest children, those who are still in diapers, are especially susceptible to infection.

The poliovirus can infect the cells of the intestine and cause mild illness: cramps, diarrhea, or constipation. Sometimes, however, the virus slips past the intestinal barrier and into the bloodstream, where it targets motor neurons in the spinal cord, the cells that tell muscles to move. When the virus infects these cells, it destroys them, leaving people paralyzed for life.

Doctors estimate that there is one case of paralytic polio for every 300 to 1,000 mild infections.

A tale of two vaccines

Until 2000, two types of vaccines were used to inoculate Americans against polio: drops, sometimes administered in sugar cubes, made from live, weakened poliovirus, and an injected vaccine that uses the dead polio.

There are several key differences between the vaccines, but one of the most important is that the oral vaccine induces so-called mucosal immunity, so if a vaccinated person comes into contact with the poliovirus again, it cannot make copies of itself. yourself in your intestine, and will not spread to another person.

However, the use of the oral vaccine also has a drawback.

“The big drawback of the oral polio vaccine is that you can expel the virus,” says Dr. James Campbell, a pediatrician and vaccine researcher at the Center for Vaccine Development and Global Health at the University of Pennsylvania School of Medicine. Maryland.

Very rarely, about one in three million times it is given, the weakened virus in the oral vaccine can escape from the intestine and cause paralysis.

The weakened virus can also be shed in the stool and, on rare occasions, mutate back into a form of the virus that can cause paralysis, especially if the virus is spread in places with poor sanitation and low vaccination rates.

“So while we were preventing polio with this vaccine, we were also rarely creating vaccine-associated polio,” said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University in Nashville.

This is what happened to the young adult from New York. Genetic sequencing showed that the virus that paralyzed him came from an oral vaccine, which is still used in other countries.

The injected vaccine, which uses the killed virus, cannot be changed into a harmful form. Although the oral vaccine is relatively safe, the injected one is even safer.

In 2000, public health authorities decided that only injections, which contain inactivated virus, should be used in the US to vaccinate against polio.

Some vaccinated people can spread the virus

However, the use of the injected vaccine has a drawback. Although it prevents paralysis, it does not necessarily prevent infection.

Therefore, young adults and children who have been vaccinated since the switch can continue to be infected with poliovirus in their intestines and shed the virus in their feces.

“They are protected against a paralytic disease, but they can still harbor the virus and spread it to others. And that’s the circumstance we have now in New York,” says Schaffner.

“So you could have essentially the entire community carrying this virus in their gut, but they don’t even know it’s there.”

That’s not a big deal if everyone around them is also protected, says Schaffner. But the fear is that silent transmission could bring the virus into pockets of people who haven’t been vaccinated against polio, and they could have more serious outcomes.

“In under-vaccinated communities, especially when there are a lot of people living in the same place who are not vaccinated, there is an opportunity for the virus to spread and spread from person to person more often,” says Campbell.

One group that could be in the risk category is that of minors. Children typically receive four polio vaccines before the age of 6: one at 2 and 4 months, a third between 6 and 18 months, and a fourth between 4 and 6 years.

Schaffner says that children who are up to date on their vaccinations, but not yet fully vaccinated, may be at higher risk for polio, but no one really knows.

“The answer would be, frankly, that they are partially protected,” Schaffner said.

“It’s the full series that gives you total protection,” he said. “We are nervous for children who are in the vaccine progression, but are not yet old enough to receive all of their vaccinations.”

Push for more vaccines

In London, where poliovirus has been detected in sewage but has yet to be diagnosed in one person, health authorities decided to give an extra dose of the vaccine to all children aged 1 to 9, just in case.

New York officials say vaccinating everyone is key to ensuring the virus doesn’t cripple more Americans.

“Our one polio case could be the tip of the iceberg, we don’t know,” Rockland County Executive Ed Day said in a video posted on Facebook. “As you can see, that could turn into a small forest fire.”

“That only happens if people don’t get vaccinated,” he said.