But there’s a reason a single case puts health officials on alert. The unvaccinated and unvaccinated are at risk for serious disease, but the spread may not be apparent — in part because of a vaccine change that happened more than two decades ago. Vaccinated people are not at risk of contracting the polio virus, which is spread from the human intestinal tract through the fecal-oral route: a person gets faecal germs on their hands, touches something, or shakes hands with another person, and that person puts the his contaminated hands to their nose or mouth. This is why younger children – those still in diapers – are particularly susceptible to infection. The polio virus can infect cells in the gut and cause a mild illness — cramps, diarrhea or constipation. Occasionally, however, the virus will slip past the intestinal 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 the year 2000, two types of vaccine were used to inoculate Americans against polio — vaccine drops, sometimes given in sugar cubes, made with live, weakened polio virus and an injectable vaccine that uses 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 comes into contact with the polio virus again, it cannot make copies of itself in the his gut and won’t pass to someone else. However, there is a downside to using the oral vaccine.
“The big downside of the oral polio vaccine is that you throw it away,” said 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 in 3 million times it is given, the weakened virus in the oral vaccine can escape from the gut and cause paralysis.
The weakened virus can also be shed in the feces, and rarely, it will mutate and change back into 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 myelitis from polio,” 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 injectable vaccine, which uses the killed virus, cannot change back into a harmful form. While the oral vaccine is relatively safe, the injectable vaccine is even safer. In 2000, public health officials decided that the US should only use vaccines, which contain the inactivated virus, for polio vaccination.

Some vaccinated people can spread the virus

However, there is a downside to using the injectable vaccine. While it prevents paralysis, it does not necessarily prevent infection. Because of this, young adults and children are vaccinated, as the switch can become infected with polio virus in their intestines and shed the virus in their feces. “They are protected from a crippling disease, but they can still harbor the virus and spread it to others. And that’s the situation we have now in New York,” says Schaffner. “So you could basically get the whole community carrying this virus in their guts, but they don’t even know it’s there.” That’s not much of a problem if everyone around them is protected, too, Schaffner says. But the fear is that silent transmission could carry the virus into pockets of people who have not been vaccinated against polio and could end up with more serious outcomes. “In very unvaccinated communities, especially when there’s a lot of people living in the same place who aren’t vaccinated, it just gives the virus an opportunity to shed and pass from person to person more often,” Campbell says. One group that could be in the risk category is children. Children usually get four polio vaccines before age 6. They give shots at 2 months, 4 months, a third shot between 6 and 18 months, and the fourth shot should be given sometime 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 increased risk of polio infection, but no one really knows. “The answer would be, frankly, they’re partially protected,” Schaffner said. “It’s the full range that gives you full protection,” he said. “We’re nervous about the kids who are in vaccine development, but they’re not yet old enough to get all the shots.”

Push for more vaccinations

In London — where the polio virus has been detected in sewage but not yet diagnosed in a person — health officials decided to give all children ages 1 to 9 an extra dose of vaccine, just in case. Officials in New York say vaccination for all is key to ensuring the virus doesn’t disable more Americans. “The 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 on Facebook. “As you can see, this could turn into a small fire.” “That only happens if people don’t get vaccinated,” he said.