One case — and sewage evidence of more spread — may not sound significant, and Americans are highly vaccinated against polio, meaning most people are protected from paralysis. According to the latest data from the U.S. Centers for Disease Control and Prevention, more than 92% of children in the United States have received polio shots by age 2.
But there’s a reason a single case alerts health officials. Unvaccinated and undervaccinated are at risk for serious illness, but the spread may not be clear — in part because of a vaccine switch that happened more than two decades ago.
Vaccinated people are not at risk if they contract poliovirus, which spreads from the human intestinal tract via the fecal-oral route: a person gets stool germs on their hands, touches something or shakes another’s hand, and that person puts their infected hands in their nose or mouth. This is why younger children – who are still in diapers – are particularly prone to infections.
Poliovirus can infect cells in the gut and cause mild illness – cramps, diarrhea or constipation. Occasionally, however, the virus slips past the gut barrier and into the bloodstream, where it lodges on motor neurons in the spinal cord — the cells that tell muscles to move. When the virus infects these cells, it destroys them, paralyzing people for life.
Doctors estimate that there is one case of paralytic polio for every 300 to 1,000 mild infections.
A story about two vaccines
Until the year 2000, two types of vaccines were used to inoculate Americans against polio: vaccine drops, sometimes given on sugar cubes, made with live attenuated polio virus, and an injected vaccine using killed polio virus.
There are several key differences between the vaccines, but a big one is that the oral vaccine induces so-called mucosal immunity, so that if a vaccinated person ever comes into contact with polio virus again, it cannot make copies of itself in their gut, and is not passed on. to another.
However, there is also a drawback to using the oral vaccine.
“The big downside of the oral polio vaccine is that you lose it,” says Dr. James Campbell, a pediatrician and vaccine researcher at the University of Maryland School of Medicine’s Center for Vaccine Development and Global Health.
Very rarely, about once every 3 million times it is administered, the attenuated virus in the oral vaccine can escape from the gut and cause paralysis.
The attenuated virus can also pass in the feces, and rarely will it mutate and go back to a form of the virus that can cause paralysis, especially if the virus is transmitted where there is poor sanitation and low vaccination rates.
“So while we prevented polio with this vaccine, we also rarely created vaccine-associated polio myelitis,” said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University in Nashville.
This is 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 turn back into a harmful form. While the oral vaccine is relatively safe, the injected vaccine is even safer.
In 2000, public health officials decided that the US could only use shots containing the inactivated virus to vaccinate against polio.
Some vaccinated people can spread the virus
However, there is a drawback to using the injected vaccine. While it prevents paralysis, it does not necessarily prevent infection.
This allows young adults and children who have been vaccinated since the switch to still become infected with poliovirus in their gut and pass the virus in their stools.
“They are protected against 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,” Schaffner said.
“So you could essentially have the whole 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 protected too, Schaffner says. But the fear is that silent transmission could bring the virus to groups of people who have not been vaccinated against polio, and could have more serious consequences.
“In highly unvaccinated communities, especially when there are a lot of people living in the same place who haven’t been vaccinated, it just allows the virus to pass from person to person more often,” Campbell says.
One group that could fall into the risk category are children. Children usually receive four polio vaccines before they are 6 years old. They get injections at 2 months, 4 months, a third injection between 6 and 18 months and the fourth injection should be somewhere between 4 and 6 years old.
Schaffner says that children who are up to date on their vaccines, but not yet fully vaccinated, are at increased risk for polio infection, but no one really knows.
“The answer, frankly, would be that they are partially protected,” Schaffner said.
“It’s the full series that gives you full protection,” he said. “We’re nervous about kids who are in vaccine progression but aren’t old enough to get all the vaccines yet.”
Push for more vaccinations
New York officials say vaccination for everyone is key to ensuring the virus doesn’t wipe out more Americans.
“Our single case of polio could be the tip of the iceberg, we don’t know,” Rockland County Executive Ed Day said in a video posted to Facebook. “As you can see, that could turn into a bit of a wildfire.”
“That only happens if people don’t get vaccinated,” he said.