Polio is back in the United States. How did that happen?

The CDC announced this week that the United States was back, baby. Back on the list of countries with circulating polio that is. Low vaccination rates in some places have allowed the virus to find a toehold in a country that declared the virus eliminated in 1979. And the United States isn’t alone among countries that had previously announced “mission accomplished” over polio. The virus has also turned up in environmental samples in the United Kingdom and in a human case in Israel.

The CDC made its polio announcement after reporting a case of a paralytic patient in New York along with positive environmental samples. Now that polio, which can permanently disable and even kill people, is back in the news, it’s worth asking what exactly is going on.

Where is polio still endemic? In 1988, fresh off the heels of eliminating smallpox, the devastating disease that plagued humanity since at least the days of the Egyptian mummies, the World Health Assembly, the WHO’s governing body, began a campaign to eliminate polio. Polio was circulating in most of the world’s countries then, and in 1988 alone there were an estimated 350,000 cases, according to the CDC. But governments had the tools to defeat the virus: incredibly effective and cheap vaccines that could offer perhaps a lifetime of protection.

For years governments around the world notched big gains against wild polio, the long circulating strains of the virus that existed before vaccines. These strains contrast with so-called “vaccine-derived poliovirus“–weakened vaccine virus in sewage that has sometimes mutated into a virulent form of the disease. Two of the three strains of wild poliovirus have been eliminated, but the campaign never quite crossed the finish line.

In both Pakistan and Afghanistan, wild polio is still considered an endemic disease—though enormous strides have been made in both countries. In the 1990s, polio disabled some 20,000 children a year in Pakistan. By mid-summer of this year, seven cases of wild poliovirus infection had been reported. In both countries, Taliban groups have at times thwarted vaccinations. Disinformation has also bred distrust of the vaccine drives. Some in Pakistan fear they are part of a campaign to sterilize Muslim children, and flyers there in 2012 claimed polio vaccinators were really American spies. But there are signs the fight against polio could be gaining momentum. The Taliban in Afghanistan agreed to restart vaccinations last fall, signing on to a plan to vaccinate millions of children against the disease.

Where is polio making a comeback? Outside of Pakistan and Afghanistan, the biggest polio threat comes not from wild-type polioviruses, but from cases derived from the use of oral polio vaccine. In fact, while wild-type polio numbers have declined, the number of vaccine-derived poliovirus cases have increased. In 2020, there were nearly 1,000.

Getting an oral polio vaccine, usually in the form of drops, doesn’t give a person polio. The drops contain a weakened form of one or more strain of the live virus. By delivering the vaccine to the gut–where the virus lives–the drops are incredibly effective and can even stop people from spreading polio. The downside, which is also kind of a plus, is that the weakened virus sheds when people poop. While this can help spread the vaccine’s protection to others, eventually the weakened virus, as it spreads from unvaccinated person to unvaccinated person, can mutate back into a virulent form. When there are enough unvaccinated targets, the strengthened virus can attack, potentially sparking an outbreak. Conversely, the injectable polio vaccine, the one given in the United States and many developed countries, doesn’t contain live virus and can’t start an infection chain. But the injection won’t stop transmission of the virus; instead it protects the vaccinated against disease.

In July, New York authorities reported a case of vaccine-derived paralytic polio in the United States, the first such report in a decade. The virus has also been found in wastewater samples in five New York counties; the samples found there were genetically related to samples from London and Jerusalem. These signs all point to hidden transmission of the virus. In Rockland County, where a case of paralysis was reported, polio vaccination rates hover around 60 percent and in some areas are as low as 37 percent, according to CNN. With no vivid memories of polio epidemics past, some people might not feel the urgency to get vaccinated.

This week, the CDC announced the United States was now on a list of some 30 countries where vaccine-derived polio is circulating. Of these countries, Nigeria has seen the most explosive growth in cases. After the WHO declared Nigeria free of wild polio in 2020, vaccine-derived cases began a rapid rise from 18 in 2019 to 415 in 2021. A report by the Independent Monitoring Board of the Global Polio Eradication Initiative, the WHO led polio campaign, cites COVID-19 disruptions and increasing instability as hampering the vaccination campaign there.

Across the continent from Nigeria, a girl in Malawi contracted wild poliovirus. Worryingly, a genetic analysis showed that it was related to a sample found in Pakistan in 2019, indicating some level of global spread of wild polio.

What were polio epidemics of the past like? There’s a good chance that many people, at least in the United States and other developed countries, associate polio with antiquated “iron lung” ventilators and other iconography of the past. After all, the wild-type disease has been eliminated in the United States since 1979.

But before that, polio outbreaks had devastating effects. In the 1950s, polio disabled about 35,000 people a year in the United States. The virus “seemed to peak” in the summer and parents feared letting their kids play outside or in swimming pools. Former President Franklin Roosevelt contracted the disease as a 39-year-old on vacation in 1921, leaving him mostly wheelchair bound for the rest of his life. And Senate Minority Leader Mitch McConnell said he still has trouble on stairs, decades after receiving physical therapy for polio when he was a toddler.

Jonas Salk, a researcher in Pittsburgh, developed the first polio vaccine in 1955, an injection of dead virus. Within years, polio rates in the United States and elsewhere began to plummet. Manufacturing problems, though, hampered uptake of Salk’s vaccine, and other products soon followed, including Albert Sabin’s oral vaccine, which contained weakened versions of the three wild poliovirus strains. The oral vaccine became the main tool in the global fight against polio. Where there were once thousands of cases of paralytic polio each year in the United States, in 2020 there were none.

Now the World Health Organization has set a target of eliminating polio by 2026. Continuing cases of wild polio in some countries—despite years of effort—show how hard that will be. Positive samples in places long thought to be done with polio are another troubling sign of apathy towards vaccination.

“For those of us that grew up at a time when we saw the adverse outcomes of pertussis and mumps and rubella and polio, it’s a lot easier to convince parents to vaccinate. When you stop seeing bad outcomes, you can easily get complacent and think ‘Why do I want to sully my child’s body with a foreign chemical?’” Yale School of Medicine Professor Howard Forman told a university outlet.

Polio, the devastating disease we never really left in the past, has apparently come roaring back. And the signs of it in New York and elsewhere point to new front in the war against the virus: complacency.