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 evidence in sewage of further spread – may not seem significant, and Americans are highly vaccinated against polio, which means most people are protected against paralysis. According to the latest data from the US Centers for Disease Control and Prevention, more than 92% of children in the US have been vaccinated against poliomyelitis by age 2.
But there’s a reason a single case puts health officials on high alert. Unvaccinated and under-vaccinated people are at risk of severe disease, but the spread may not be obvious – in part because of a vaccine switch that happened more than two decades ago.
Vaccinated people are not at risk if they catch the poliovirus, which spreads from the human intestinal tract through the fecal-oral route: a person gets germs from stool on their hands, touches something, or shakes hands with someone. another person, and that person puts their hands in their nose or mouth. This is why young children – those still wearing diapers – are particularly susceptible to infections.
Poliovirus can infect cells in 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 lodges in motor neurons in the spinal cord – the cells that tell muscles to move. When the virus infects these cells, it destroys them, leaving people with lifelong paralysis.
Doctors estimate that there is one case of paralytic poliomyelitis for every 300 to 1,000 mild infections.
Until 2000, two types of vaccines were used to immunize Americans against polio: vaccine drops, sometimes given on sugar cubes, which were made with live, weakened poliovirus, and an injected vaccine that uses poliovirus kill.
There are several key differences between the vaccines, but the most important is that the oral vaccine induces so-called mucosal immunity, so that if a vaccinated person comes into contact with poliovirus again, it cannot replicate in their gut. and will not be passed on to anyone else.
However, there is also a downside to using the oral vaccine.
“The big downside to the oral polio vaccine is that you lose it,” said Dr. James Campbell, a pediatrician and vaccine researcher at the Center for Vaccine Development and Global Health at the University of Maryland School of Medicine. .
Very rarely, about once in 3 million times it is given, the weakened virus from the oral vaccine can escape from the gut and cause paralysis.
The weakened virus can also shed in the stool, and rarely, it will mutate and revert back to a form of virus that can cause paralysis, especially if the virus is transmitted where there is poor sanitation and low vaccination rates.
“So while we were preventing polio with this vaccine, we were also rarely creating polio-associated myelitis,” said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University in Nashville.
That’s what happened to the young adult in 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 become harmful again. While the oral vaccine is relatively safe, the injected vaccine is even safer.
In 2000, public health officials decided that the United States should only use vaccines containing inactivated virus to vaccinate against poliomyelitis.
However, there is a downside to using the injected vaccine. Although it prevents paralysis, it does not necessarily prevent infection.
For this reason, young adults and children vaccinated since the switch can still be infected with poliovirus in their intestines and shed the virus in their stool.
“They are protected against a paralytic disease, but they can still harbor the virus and pass it on to others. And that’s the situation we have now in New York,” Schaffner says.
“So you could have pretty much the whole community carrying this virus in their gut, but they don’t even know it’s there.”
It’s not a big deal if everyone around them is equally protected, Schaffner says. But the fear is that silent transmission could spread the virus to pockets of people who have not been vaccinated against polio, and they could end up with more serious outcomes.
“In heavily unvaccinated communities, especially when there are a lot of people living in the same place who are unvaccinated, it just gives the chance for the virus to spread and be transmitted more frequently from person to person. ‘other,” says Campbell.
One group that might be in the at-risk category is children. Children usually receive four polio vaccines before the age of 6. They receive injections at 2 months, 4 months, a third between 6 and 18 months and the fourth between 4 and 6 years.
Schaffner says children who are up to date on their vaccines, but not yet fully immunized, may be at increased risk of polio infection, but no one really knows.
“The answer would be, frankly, they’re partially shielded,” Schaffner said.
“It’s the full series that gives you full protection,” he said. “We are concerned for children who are in the vaccine progression, but have not yet been old enough to receive all the vaccines.”
In London – where poliovirus has been detected in sewage, but not yet diagnosed in a person – health officials have decided to give an extra dose of vaccine to all children from 1 to 9 years old, just in case.
New York officials say getting everyone vaccinated is key to ensuring the virus doesn’t disable more Americans.
“Our one case of polio could be the tip of the iceberg, we don’t know that,” Rockland County Executive Ed Day said in a video posted to Facebook. “As you can see, this could turn into a wildfire.”
“It only happens if people aren’t vaccinated,” he said.