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Polio 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. A decades-long push to increase immunizations and access to clean water has meant that mass outbreaks, with hospital wards filled with children kept alive in iron lungs, are a distant memory. But while polio was brought to the brink of global eradication, it’s been bouncing back, radiating out from reservoirs in Pakistan and Afghanistan, where conflict and resistance to vaccination have kept the virus circulating. The disruptions of the Covid-19 pandemic and one vaccine’s weaknesses have played a role. Outbreaks in 2022 in the US, the UK and Israel, which had all eliminated polio decades earlier, showed that nowhere was safe and added urgency to the effort to round up funds for a renewed push.

Poliomyelitis is a highly infectious disease caused by one of three poliovirus types. Once inside the body, the virus can invade the nervous system and cause paralysis in a matter of hours. In other 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, a fact that makes it more difficult to prevent transmission, which usually takes place via fecal matter. About one case in 200 leads to irreversible paralysis, usually in the legs. Among those paralyzed, 5% to 10% die when their breathing muscles become immobilized. There is no cure for paralytic polio and no specific treatment. Polio mainly affects children younger than 5, though anyone who is unvaccinated can contract it. 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 as long as 15 to 40 years after a patient’s recovery.

2. How many people does polio affect?

Cases resulting in paralysis have declined sharply since vaccines became widely available, and especially since 1988, when a vaccination campaign, the Global Polio Eradication Initiative, began. Back then, polio was paralyzing more than 1,000 children daily across 125 countries. In 2016, there were just 42 polio cases reported, a low, but the number had climbed to 697 by 2021. Six of those were caused by wild poliovirus. The rest were inadvertently caused by a variant of the virus used in the oral polio vaccine.

3. How does the vaccine cause polio cases?

The vaccine developed by Albert Sabin in the 1950s uses a live, attenuated form of the virus. Among its advantages: Children who receive the inexpensive immunization, which is delivered orally, 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 circulates over a prolonged period in an undervaccinated community, it can undergo genetic changes that restore its paralyzing ability. A second polio vaccine, originally developed by Jonas Salk, is routinely given as a shot in more than 120 countries, including the US. It contains inactivated or dead poliovirus that can’t cause paralysis. A new oral vaccine less prone to mutations began to be rolled out in Africa in 2020.

4. Why is polio rebounding?

The $19 billion global campaign eradicated the virus from much of the globe. But it has retained a foothold in Pakistan and Afghanistan, areas of continuing conflict with a history of suspicion of vaccination programs that was amplified by the US Central Intelligence Agency’s use of a vaccination worker to gather information that led to the killing of Osama bin Laden in 2011. From that tenacious stronghold, the wild poliovirus spread back into areas where Islamist militants were active, including Syria and Iraq, and from the Middle East radiated to southeastern Africa. Over the past six years, those reservoirs have seeded outbreaks of imported poliovirus in dozens of countries. In 2022, floods that inundated much of Pakistan led to an increase in the disease’s circulation.

5. What effect has Covid-19 had?

In 2020, the pandemic prompted a four-month pause of the 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. Cases declined in 2021 as immunizations resumed. However, the pandemic continues to stretch health systems. Governments dealing with financial strains in Covid’s wake have been slow to respond to a call by the eradication campaign for $4.8 billion in new funds.

(An earlier version of this story corrected a math error in section 2)

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