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Why Polio, Once Nearly Eradicated, Is Rebounding

Poliomyelitis (“polio” for short) has been a cause of life-threatening paralysis for thousands of years. At the height of the biggest-ever outbreak in 1952, almost 60,000 cases with more than 3,000 deaths were reported in the US alone. Thanks to immunizations and access to clean water, wards filled with children kept alive in iron lungs are a distant memory. For years, polio has teetered close to global eradication. Now a combination of old and new challenges -- including conflict, vaccine hesitancy and the Covid-19 pandemic -- are frustrating a decades-long effort to stop its international spread. A case in an unvaccinated man from New York in July was a reminder that failure to vanquish polio from its last remaining strongholds could result in a resurgence of the crippling disease.

1. What’s polio?

It’s a highly infectious disease caused by one of three poliovirus types that replicate in the human gastrointestinal tract. Infectious viral particles are shed in fecal matter, which can infect other people if they are transferred to the mouth via unwashed hands or ingested in contaminated food and drinks. Virus particles can survive in soil and water for months; the duration is shortened by warm weather and sunlight, and formaldehyde and chlorine kill them. Once inside the body, the virus can invade the nervous system and cause paralysis in a matter of hours. In some cases, symptoms can take as many as 30 days to appear. Initial signs include fever, fatigue, headache, vomiting, stiffness of the neck and pain in the limbs. Most infections, though, are asymptomatic, with only 1-in-200 leading to irreversible paralysis, usually in the legs. Among those paralyzed, 5% to 10% die when their breathing muscles become immobilized. Polio mainly affects children younger than 5, though anyone who is unvaccinated can contract the incurable disease. In the long term, 25% to 40% of children who recover from paralytic polio get post-polio syndrome, a group of potentially disabling symptoms including weakness and fatigue that appear some 15 to 40 years after a patient’s recovery. 

2. How many people does polio affect?

Cases of paralytic polio have declined sharply since vaccines became widely available, and especially since 1988, when health officials began a campaign, the Global Polio Eradication Initiative, that initially sought to put an end to the disease within 12 years. Back then, polio was paralyzing more than 1,000 children daily across 125 countries. Although only six cases caused by wild poliovirus were reported in 2021, 10 times more were inadvertently caused by a variant of the virus used in the oral polio vaccine.

3. How does the vaccine cause polio cases?

The oral polio vaccine developed by Albert Sabin in the 1950s uses a live, attenuated form of the poliovirus. Among its advantages: Children who receive the inexpensive immunization excrete the virus in their stools for as long as six weeks, passively “vaccinating” those around them. A disadvantage is that if the weakened virus is transmitted from person to person over a prolonged period in an under-vaccinated community, it can undergo genetic changes that turn it back into the paralysis-causing form. A second polio vaccine, originally developed by Jonas Salk, is routinely given as a shot more than 120 countries, including the US. It contains inactivated or dead poliovirus that can’t cause paralysis. It’s usually administered to kids four times from age 2 months to 6 years. 

4. What’s known about the New York case?

The 20-year-old man, who was hospitalized in June, was a resident of Rockland County, which is north of New York City and bordering New Jersey. He contracted a poliovirus variant derived from the oral vaccine that’s been genetically linked to strains collected from wastewater in the county as well as London and the greater Jerusalem area. The patient had recently traveled to Europe, according to the Washington Post. New York authorities have been investigating the source of the infection and testing wastewater to assess the virus’s spread. (All polioviruses are excreted in feces, so traces of the pathogen are often detected in sewage when enough infected people are shedding virus in the area of a particular wastewater system.) Since only the inactivated polio vaccine has been given in the US since 2000, it’s likely the virus originated somewhere else where the oral vaccine is still used. Rockland County has a polio vaccination rate of 60.5% among 2-year-olds, compared with the New York average of 79.1%. Low rates of routine pediatric vaccination within communities in the New York metropolitan area were linked to a 2018-2019 measles outbreak, the largest in the US since 1992. The nation’s last naturally occurring cases of polio were reported in 1979. The last known case in the US derived from the oral vaccine was recorded by the Centers for Disease Control and Prevention in 2013. 

5. Why is polio rebounding?

Outbreaks of imported poliovirus, like those seen in the US, UK and Israel in 2022, have occurred in dozens of other countries over the past six years, and for the same reason: undervaccination. There are multiple reasons for low immunization rates, according to virologist Mark Pallansch, a senior advisor at the CDC. In general, vaccine hesitancy and avoidance -- reinforced by disinformation -- yield similarly undervaccinated populations, Pallansch wrote in the Oct. 5 edition of the New England Journal of Medicine. Rockland has the lowest routine childhood immunization coverage of any county in New York, The Lancet medical journal reported in August. In London, the proportion of infants immunized with the inactivated polio vaccine has fallen in recent years, dropping as low as 73.6% in some boroughs -- below the 80% thought to be needed to stop transmission in a community.

6. What effect has Covid-19 had?

In 2020, the pandemic prompted a four-month pause of the Global Polio Eradication Initiative’s campaigns, disrupting disease surveillance and routine immunizations. That put more than 80 million children at increased risk of polio. Vaccine-derived poliovirus outbreaks tripled from 2019 to 2020, when more than 1,100 children were paralyzed globally. Cases declined in 2021 as immunizations resumed. However, the pandemic continues to stretch health systems, risking further spread. In late 2021, wild poliovirus from Pakistan -- which along with Afghanistan hasn’t been able to stop transmission of the virus -- sparked outbreaks in Malawi and Mozambique. The humanitarian crisis in Afghanistan following its change in political regime and the war in Ukraine, where weak immunization has repeatedly given rise to outbreaks of vaccine-derived poliovirus, threaten to prolong the scourge. The 32nd meeting of an emergency committee convened by the World Health Organization concluded in June that the risk of international spread remains a public health emergency -- a declaration first made in 2014.

7. Who’s at risk of polio?

Anyone who hasn’t received the recommended doses of polio vaccine, including babies. Almost all children (99 out of 100) who complete an immunization course will be protected from polio. In respect to the Rockland County case, unvaccinated New Yorkers who live, work, go to school in, or visit the county are at the highest risk of exposure, according to the state’s health department. Although vaccination isn’t necessary for most adults who were immunized against polio as children and have a low risk of exposure, the CDC recommends a one-time booster shot for certain groups:

• Travelers to areas or countries where polio is epidemic or endemic

• Health-care workers who have close contact with patients who may have traveled to areas or countries where the risk of polio is greater

• Unvaccinated adults whose children will be receiving oral poliovirus vaccine (for example, international adoptees or refugees)

These higher-risk adults may need one to three doses of the inactivated polio vaccine, depending on how many doses they have had in the past. Unvaccinated adults at risk for poliovirus infection should get three doses: two doses separated by 1 to 2 months, and a third dose 6 to 12 months after the second dose.

8. Is a booster really necessary?

Immunity provided either by the oral vaccine or exposure to poliovirus provides lifelong protection against paralytic disease, according to a WHO position paper. It’s not known how long people who received four shots of inactivated polio vaccine will be immune, but the CDC says they’re most likely protected for many years and the WHO says possibly for life. Higher-risk adults who have had one or two doses of polio vaccine in the past should get the remaining doses, and higher-risk adults who have had three or more doses of polio vaccine in the past can get a lifetime booster dose, according to the CDC. 

(Adds detail on wastewater surveillance in section 4 and undervaccination in section 5, and section 6 on Covid’s impact.)

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