A recent spate of polio-related news in the U.S. has left the public wondering: Is polio back? The short answer is yes—but a high-profile New York State outbreak is related to unusual factors that don’t apply to the general population. Measures to address pockets of dangerously low polio vaccination rates around the U.S. could extinguish the outbreak and potentially restore the country to a poliovirus-free condition.

Experts agree that a vaccinated population is the best defense against poliomyelitis, the technical name for the disease caused by the poliovirus—which can sometimes lead to permanent paralysis. Fortunately, U.S. polio vaccination rates have exceeded 90 percent in recent years, with most children receiving three doses by age two, as recommended by the U.S. Centers for Disease Control and Prevention. A man in Rockland County, New York, is the sole person to have developed polio in the U.S. in almost a decade.

But public health workers subsequently detected genetic signs of the virus in wastewater samples in New York City—as well as in Rockland County and three other counties in the state—suggesting other people in the region may have been infected, too. In three of the affected counties, polio vaccination rates hover around 60 percent, leaving unvaccinated residents vulnerable. And many people infected with the virus never develop symptoms but can still pass the virus to others.

To prevent the stealthy spread of polio in communities, New York Governor Kathy Hochul issued an emergency declaration this month that adds pharmacists, paramedics and midwives to the list of people the state authorizes to administer the vaccine.

The type of poliovirus now circulating in the U.S. complicates polio eradication here and reflects the history of vaccination against the condition. In the last few decades of the 20th century, the U.S. relied on an oral polio vaccine containing a weakened version of the virus. The oral vaccine is cost-effective, easy to administer, involves no needles and triggers an immune response that reduces risk for later infection and prevents disease. It comes with an additional benefit: a person carrying the weakened virus can shed it for a few weeks, passing it to unvaccinated close contacts without causing symptoms—a form of secondary immunization. The upshot is amplification, meaning a single, easily administered vaccination can indirectly immunize many people against both infection and disease once immunity kicks in.

But in sporadic cases, the weakened virus has led to polio in vaccine recipients. From 1980 to 1992, 109 cases of this vaccine-related polio occurred with the administration of 262 million doses of the oral vaccine in the U.S., for an average of about 8.4 cases annually. In response, and because of wild polio’s eradication from the country, in 2000 the U.S. switched to an injectable vaccine that uses a killed virus and does not carry this risk. The killed-virus vaccine protects against polio itself but not against viral transmission.

In regions around the world where the oral vaccine remains in use, the weakened virus can circulate beyond secondary immunization of immediate contacts and travel through unvaccinated or undervaccinated populations. As it does so, it can accumulate mutations and eventually emerge as a symptom-causing form known as vaccine-derived poliovirus. Unvaccinated people exposed to this reinvigorated pathogen are susceptible to developing paralytic disease, which occurred in the Rockland case. Vaccine-derived poliovirus can crop up in the U.S. if travelers pick it up elsewhere and bring it back to the country. Vaccinated people remain protected against paralytic polio caused by either the vaccine-derived or wild versions of the virus.

Two weeks ago the U.S. achieved the dubious distinction of joining the World Health Organization’s list of countries with circulating vaccine-derived poliovirus. The single polio case in Rockland County, along with detection of vaccine-derived virus in wastewater, earned the country’s place on the roster.

For insights into the current status of polio in New York and elsewhere in the U.S., Scientific American spoke with infectious disease experts. They offered crucial information about who is at risk, who might need boosters, why vaccination is the most important preventive against polio, and how vaccine-derived and wild polioviruses differ.

The polio case in New York, along with detection of the virus in area wastewater, sounds significant. Are people in the U.S. now at risk of paralytic polio?

Not if you’re vaccinated. “This is an unfortunate event because it is entirely preventable,” says Yvonne Maldonado, a pediatric infectious disease physician and chief of the division of pediatric infectious diseases at the Stanford University School of Medicine.

Wild poliovirus causes polio unrelated to vaccination. No cases involving wild poliovirus have originated in the U.S. since 1979, according to the CDC, and there have been no cases related to wild poliovirus in the country at all since 1993. “We know it’s gone because we’ve looked for it and haven’t found it,” Maldonado says.

Thanks to decades of successful vaccination campaigns, wild poliovirus cases dropped from hundreds of thousands worldwide in 1988 to only six in 2021, according to the WHO. Global eradication of polio was within reach, but geopolitics in Afghanistan and Pakistan, where the wild virus still lingers, has hindered this important public-health goal. The two countries have reported 21 cases of wild poliovirus so far in 2022.

Many parents skipped taking their children to a pediatrician for regular wellness checks in the first year or two of the COVID pandemic, raising concerns about sliding vaccine rates. New York’s emergency declaration expands access to the polio vaccine and could thus help halt that slide, says Amesh Adalja, an infectious disease physician and senior scholar at the Johns Hopkins Center for Health Security. Along with raising public awareness, the declaration “allows broad scope of practice to allied health professionals to be able to vaccinate individuals,” Adalja says.

For most people in the U.S., even in New York, the risk of contracting vaccine-derived poliovirus is low. It is safe to travel to the state, Maldonado says. Poliovirus is largely transmitted by the unwitting ingestion of fecal matter—so beyond full vaccination, “normal handwashing and normal sanitation will really keep you safe,” she says.

Do risks vary with age?

People who get infected with poliovirus in adolescence or later tend to fare worse than babies do, Maldonado and Adalja say, which is another reason to vaccinate early and get protection in place. The CDC childhood polio vaccine schedule calls for a series of three shots by 18 months of age and a final one around the time that children enter kindergarten.

How can people find out if they are already vaccinated?

Most people in the U.S. are vaccinated against polio, Maldonado says, but decades-old records can be hard to locate because the country lacks a national tracking system. People who are concerned should consult their doctors, she says.

As of 2016, 92.5 percent of children had received the recommended three doses of polio vaccine by 24 months of age, according to the CDC. That is the highest rate for any of the listed childhood vaccines. Up to 97 percent of U.S. residents had antibodies to wild polioviruses, according to a CDC analysis of data collected in 2009 and 2010. This finding held up in the subset of people who would have received the oral vaccine in childhood decades ago. At these rates, vaccine-derived poliovirus will hit a wall of immunity in most of the country.

New York’s Rockland County is one of the exceptions, however. In part because of an active antivaccine movement in the community, its rate of polio vaccination by age two is 60.3 percent, the third lowest in the state. At that level, “you start to see what happens when people just let go of their vaccine efforts,” Maldonado says. “We are victims of our own success in this country because people don’t think they need to worry anymore.”

How effective is the polio vaccine?

People vaccinated with the killed-virus vaccine can be infected by and transmit the virus to others but will not develop symptoms. Vaccination with the weakened form triggers a stronger immune response than the killed-virus vaccine, one that reduces transmission risk with later virus exposures. Both vaccines are administered on a similar schedule, and both are extremely effective against symptomatic disease. In adults, a three-dose series of the killed-virus vaccine is 99 to 100 percent effective, the CDC says—and children get that fourth dose, too. Maldonado says the number of shots varies with age because of different immune responses between children and adults.

How safe is the polio vaccine?

The current vaccines administered in the U.S., with the injectable inactivated virus, “have been used for 60 years–plus, and they are incredibly safe,” Maldonado says. “The risks are reversible, and I can’t even think of a risk factor but pain at the injection site.”

Do people vaccinated decades ago need a booster against polio?

“I wouldn’t worry,” Maldonado says. “We have not seen [vaccinated] people developing paralysis, even if they are older. It’s really remarkable how protective the vaccines are against disease.”

Some experts do recommend that New York state residents in high-risk occupations—such as health care and wastewater workers—get boosted. “We usually only give an extra booster to people traveling to countries such as Afghanistan and Yemen, where there is a concern that polioviruses might be circulating,” Adalja says.

The CDC states that adults who are at high risk for polio exposure can receive one “lifetime” booster of inactivated polio vaccine. The agency lists some countries in Asia, Europe and Africa as destinations warranting a booster.

How do I find information about polio risk or vaccination levels where I live?

Some state health departments publish polio vaccination rates by county, which can indicate the sturdiness of the area’s wall of polio immunity. Most local public health departments post no information about polio cases—because there are none.

New York is continuing to monitor wastewater samples for poliovirus. But municipalities usually don’t do so.

“The solution is super simple: vaccinate people,” Adalja says. In places where antivaccine activists have lowered vaccination rates, “that’s also going to be a place where polio is successful,” he says. “Wherever they’re successful, only pestilence and disease will follow in their wake.”