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Polio used to disable tens of thousands of people, often children, a year. Credit: National Archives at College Park via Wikimedia Commons.

The CDC announced this week that the US is 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 foothold in a country that declared the virus eradicated in 1979. And the United States is not alone among countries that had declared previously “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 from the eradication of smallpox, the devastating disease that had plagued humanity since at least the days of the Egyptian mummies, the World Health Assembly, the governing body of the World Health Organization, began a campaign to eradicate polio . Polio was circulating in most countries of the world at that time, and in 1988 alone there were an estimated 350,000 cases, according to the CDC. But governments had the tools to defeat the virus: highly effective and cheap vaccines that could offer perhaps a lifetime of protection.

For years governments around the world have seen major gains against wild polio, the long-circulating strain of the virus that existed before vaccines. These strains are in contrast to what is known as “vaccine-derived poliovirus” – a vaccine virus diluted in sewage that has sometimes mutated into a virulent form of the disease. Two of the three types of wild poliovirus have been eradicated, but the campaign never crossed the finish line.

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

Where is polio coming back? Outside of Pakistan and Afghanistan, the greatest polio threat does not come from wild-type polioviruses, but from cases resulting from the use of oral polio vaccine. In fact, while the numbers of wild-type polio have decreased, the number of vaccine-derived poliovirus cases has increased. In 2020, there were almost 1,000.

Getting an oral polio vaccine, usually in the form of drops, does not give a person polio. The drops contain a weakened form of one or more strains of the live virus. By delivering the vaccine to the gut – where the virus lives – the drops are highly effective and can even prevent people from spreading polio. The disadvantage, which is also an advantage, is that the weakened virus disappears when people defecate. 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 stronger virus can attack, potentially triggering an outbreak. Conversely, the injectable polio vaccine, the one given in the United States and many developed countries, does not contain live virus and cannot start a chain of infection. But the injection will not prevent transmission of the virus; rather it protects those who have been 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. All these signs point to covert 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 past polio epidemics, some people may not feel the urgency to get vaccinated.

This week, the CDC announced that the United States is 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 WHO declared Nigeria free of wild polio in 2020, vaccine-derived cases began to rise rapidly 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, citing COVID-19 disruptions and growing instability in blocking the vaccination campaign there.

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

What were polio epidemics like in the past? There’s a good chance that many people, at least in the United States and other developed countries, associate polio with ancient “iron lung” ventilators and other iconography of the past. After all, the wild-type disease has been eradicated 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 be “at its peak” in the summer and parents were afraid to let their children play outside or in swimming pools. Former President Franklin Roosevelt contracted the disease as a 39-year-old man on vacation in 1921, leaving him mostly in a wheelchair for the rest of his life. And Senate Minority Leader Mitch McConnell said he still struggles on the stairs, decades after receiving physical therapy for polio as a toddler.

Jonas Salk, a researcher in Pittsburgh, developed the first polio vaccine in 1955, an injection of a dead virus. Within years, polio rates in the United States and elsewhere began to plummet. However, manufacturing problems hampered uptake of the Salk vaccine, and other products that soon followed, including Albert Sabin’s oral vaccine, which contained weaker versions of the three wild poliovirus strains. The oral vaccine became the main weapon 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 eradicating polio by 2026. Continued outbreaks of wild polio in some countries — despite years of effort — show how difficult that will be. Positive samples in areas long believed to have been infected with polio are another troubling sign of indifference to vaccination.

“For those of us who grew up at a time when we saw the harmful consequences of pertussis and mumps and rubella and polio, it is much easier to convince parents to vaccinate. When you stop seeing bad results, you can easily become complacent and think ‘Why do I want to soak my child’s body with a foreign chemical?'” Professor Howard Forman of the Yale School of Medicine told the outlet University.

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

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