One case, and evidence in sewage of further spread, may not seem significant, and Americans are highly vaccinated against polio, meaning that most people are protected from paralysis. According to the latest data from the US Centers for Disease Control and Prevention, more than 92% of US children have received polio inoculations by age 2.
But there’s a reason a single case has health officials on alert. The unvaccinated and undervaccinated are at risk of serious disease, but the spread may not be obvious, in part because of a vaccine change that happened more than two decades ago.
Vaccinated people are not at risk if they contract poliovirus, which is spread from the human intestinal tract through the fecal-oral route: a person gets germs from feces on their hands, touches something, or shakes hands with another person, and this person puts their contaminated material. hands to nose or mouth. This is why younger children, those still in diapers, are especially susceptible to infection.
Poliovirus can infect cells in the intestine and cause mild illness: cramps, diarrhea, or constipation. Occasionally, however, the virus will slip past the gut barrier and into the bloodstream, where it will lodge 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 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 of vaccine, sometimes given in sugar cubes, that were made with live, weakened poliovirus, and an injected vaccine that uses dead polio virus.
There are several key differences between the vaccines, but one of the big ones is that the oral vaccine induces so-called mucosal immunity, so that if a vaccinated person comes into contact with the poliovirus again, they cannot make copies of herself in the gut. it will not be passed on to anyone else.
However, there is also a downside to using the oral vaccine.
“The big downside of the oral polio vaccine is that it kills 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 every 3 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 will rarely mutate and change 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 were preventing polio with this vaccine, we were also rarely creating 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 originated from an oral vaccine, which is still used in other countries.
The injected vaccine, which uses the dead virus, cannot convert back into a harmful form. Although the oral vaccine is relatively safe, the injected vaccine is even safer.
In 2000, public health officials decided that the US should only use vaccines, which contain the inactivated virus, 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.
Because of this, young adults and children vaccinated since the switch can still be infected with poliovirus in their intestines and shed the virus in their stools.
“They’re protected against a paralytic disease, but they can still harbor the virus and spread it to other people. And that’s the circumstance we have now in New York,” says Schaffner.
“So you could get essentially the whole community carrying this virus in their intestines, 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 into the pockets of people who have not been vaccinated against polio, and could end up with more serious outcomes.
“In highly unvaccinated communities, especially when there are a lot of people living in the same place who aren’t vaccinated, it just gives the virus an opportunity to get off the ground and pass from person to person more often,” says Campbell.
A group that could be in the risk category are children. Children usually receive four polio vaccines before age 6. They receive the vaccines at 2 months, at 4 months, a third vaccine between 6 and 18 months, and the fourth vaccine between 4 and 6 years.
Schaffner says that children who are up to date on their vaccines, but not yet fully vaccinated, may be at higher risk of polio infection, but no one really knows.
“The answer would be, frankly, they’re partially protected,” Schaffner said.
“It’s the complete series that gives you total protection,” he said. “We’re nervous about kids who are in vaccine progression but not yet old enough to get all the vaccines.”
Push for more vaccines
In London, where poliovirus has been detected in sewage but not yet diagnosed in a person, health officials decided to give all children aged 1 to 9 an extra dose of vaccine, just in case case
New York officials say vaccination for all is key to ensuring the virus doesn’t incapacitate more Americans.
“Our one case of polio 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, this could turn into quite a fire.”
“This only happens if people are not vaccinated,” he said.