For the first time in almost a decade, a case of polio was confirmed in the United States. Health officials in New York’s Rockland County discovered the case last month in an unvaccinated 20-year-old, decades after polio was eliminated from the US in 1979.

With the country and public health system already struggling under the weight of Covid-19 and monkeypox, this news comes as an unpleasant surprise, and instantly raises questions. How did this happen? Who else is at risk? What does it mean that the Rockland case was a vaccine-derived strain, and what are the implications for the global efforts to fully eradicate polio?

What is polio?

Polio, short for poliomyelitis, is caused by the poliovirus, an enterovirus that can infect the nervous system. Symptoms can range from those similar to the flu (sore throat, fever, and fatigue), to a more severe infection of the spinal cord causing meningitis and even paralysis. But unlike the flu, the poliovirus multiplies mainly in the intestines, and it chiefly spreads when people don’t wash their hands after using the bathroom. Polio is highly contagious, at least to the unvaccinated, particularly in areas with poor sanitation and water safety.

From the first documented US outbreak in 1894 until vaccines were developed in the 1950s, polio was one of the most feared childhood diseases. Thousands of children were left paralyzed with every summer outbreak. The most vulnerable were children under the age of 5.

But those victims were the exception; three-quarters of patients infected with the poliovirus show no symptoms at all. For most of the remaining quarter, the illness never progresses beyond flu-like symptoms. In roughly one in 25 patients, however, the virus spreads to the nervous system and causes meningitis. About one in eight of the meningitis cases — or approximately 0.5 percent of total polio cases — will have permanent damage to their nerves that leaves them paralyzed. There was and is no known cure, only supportive treatments including the iron lung — since replaced by more advanced ventilators — and physical therapy.

The threat of polio changed permanently when two vaccines were discovered in short succession: an injected, inactivated vaccine by Dr. Jonas Salk in 1955, and a live-attenuated vaccine, taken orally, by Dr. Albert Sabin in 1961. Both vaccines are very effective, granting 99 percent immunity to infection. Sabin’s oral vaccine was eventually adopted widely in the US, and polio cases dropped drastically in the 1960s and 70s, until the wild virus was stamped out entirely in the country.

The US was ahead of the curve — a global vaccination campaign began in earnest in 1988, a few years after smallpox was declared eradicated in 1980. The US switched to the slightly safer inactivated, injected vaccine in 2000, and the shots are still recommended to all children on the standard childhood vaccine schedule. Worldwide, thanks to ongoing public health efforts, hundreds of millions of children receive the oral vaccine every year, and the original wild virus has been driven out of all but a handful of countries.

Where did this case come from?

Since community spread of polio was eliminated from the US around 1980, all infections have come from other countries that still have the disease. Genetic sequencing shows that the recent case was a vaccine-derived poliovirus strain. This means the circulating virus isn’t from one of the few remaining pockets of endemic wild poliovirus, but rather from one of the many more countries with polio outbreaks that mutated from an oral, live-attenuated vaccine — which is not the vaccine currently used in the US.

Polio vaccines fit into one of two types: inactivated or live-attenuated. Live-attenuated vaccines, like the combined measles, mumps, and rubella vaccine recommended to all US children, contain a modified, weakened strain of a pathogen that doesn’t cause illness in humans, but still triggers an immune response that protects against the original strain. The oral vaccine used in the most at-risk countries is live-attenuated. Inactivated vaccines, like the polio vaccine currently used in the US, contain only dead virus material, and may need a longer series of booster shots to stimulate the immune system enough to grant long-lasting and full immunity.

Although the live-attenuated poliovirus vaccine almost never causes polio itself — except in the less than one in a million cases when a child is severely immunocompromised — the fact that it contains a live virus inevitably carries some risk, unlike inactivated vaccines. When live-attenuated polio vaccines are given in a community that contains a high fraction of unvaccinated people, the modified virus can infect others, and with enough generations of spread, it can — very rarely — mutate back into a new virulent strain. It’s essential to public health efforts to make sure enough people get vaccinated, to protect against both the wild virus and the possibility of new vaccine-derived strains.

Ironically, the fact that most polio cases are asymptomatic or mild — along with an incubation period that can take up to 30 days before symptoms appear — makes polio particularly challenging for contact tracing and public health containment efforts. The only way to keep the virus suppressed is by achieving herd immunity, which for polio requires vaccinating about 80 percent of the population.

Who is at risk?

For most people in the US, the newly discovered polio case hasn’t raised the risk at all. Rockland County’s public health department believes that the patient is no longer contagious.

The poliovirus can be detected in stool samples, and also in wastewater monitoring, which looks for evidence of viral genetic material in sewage. On August 1, the New York State Department of Health reported that the Rockland polio case was genetically linked to samples of the virus collected in sewage in Jerusalem and London, though the department stressed that the results do not automatically imply the patient had traveled to either location. The Rockland public health department was able to use sewage samples collected earlier for Covid-19 monitoring, and found poliovirus in samples there from June that are genetically connected to the current case.

Given how common asymptomatic cases are and the long incubation period, it’s possible there are other unrecognized cases in the Rockland area. Those may still be infectious, but the odds are against it spreading far. As of 2019, over 90 percent of US children were fully vaccinated against polio on schedule, well above the herd immunity threshold, and this figure has held steady for decades. Infants 4 months or older will usually have received two doses, which already provides 90 percent immunity.

Rockland County, though, has a lower vaccination rate than the rest of the country; it was the site of a 2018-2019 measles outbreak, and currently only 60 percent of 2-year-olds there are fully vaccinated against polio, compared to the national average of 90 percent. The New York State Department of Health is now urging all unvaccinated people, those who haven’t completed their polio vaccine series, and pregnant people to get vaccinated. In the month since the polio case was discovered, ​​the Rockland clinic administered almost 400 vaccine doses. People in the Rockland area who were vaccinated as children but are worried they may have been exposed should schedule a booster shot.

What does this mean for the global eradication effort?

While the US remains protected against polio, the same can’t be said of some more at-risk developing countries where the virus is still active.

After his work developing the oral vaccine, Sabin campaigned for a worldwide eradication effort in the 1960s, and in 1972 donated all of his vaccine strains to the World Health Organization in the hope of reducing the manufacturing cost. Despite recent efforts to introduce the slightly safer inactivated vaccine worldwide, most lower- and middle-income countries still use the oral vaccine.

The global eradication program has been a huge success overall, with total worldwide polio cases declining by more than 99.99 percent since the program started in 1988. But the closer eradication gets, the harder reaching the finish line becomes. When hundreds of millions of doses of oral vaccine are given every year, even the very low risk of a dose spawning a new vaccine-derived strain adds up. Most of the polio cases that have been detected in African countries like Nigeria and Yemen are vaccine-derived. Interruptions in vaccination coverage due to military conflicts and the Covid-19 pandemic likely increased the risk of vaccine-derived variants spreading unchecked.

Despite the risks inherent to live-attenuated vaccines, the oral vaccine has significant advantages, particularly for public health campaigns in developing countries. Each dose costs as little as 12 cents, compared to about $2 per dose for the inactivated vaccine, and because it’s given in drops under the tongue, it doesn’t require needles or trained professionals to administer. Live-attenuated vaccines in general also provide stronger and longer-lasting immunity than inactivated vaccines.

And early on, the infectiousness of the oral vaccine strain was actually considered a plus, since children not reached by health workers could potentially catch the weakened strain from others, ending up immune. In theory, as long as the vaccination campaign reached enough people in the community, the spread would fizzle out long before the virus had a chance to mutate back to virulence in humans.

Phasing out the oral vaccine, which would eliminate the source of new polio variants, will likely be needed to reach full eradication, but replacing the oral vaccine with the full schedule of booster shots needed to grant immunity isn’t yet possible. Even if the funding and personnel were available, the total global supply of inactivated vaccines is far too low to cover the hundreds of millions of children still at risk.

With monkeypox having been recently declared a public health emergency of international concern by the WHO, and the ever-present threat of future pandemics on the horizon, the global effort against polio is more important than ever to ensure that polio will never again be that kind of worldwide threat. Maintaining and ideally increasing the vaccination rate in the US will protect the country in the meantime, and support the worldwide push for eradication by denying polio a foothold.