The polio vaccine is important because it is the only way to prevent a virus that can cause irreversible paralysis and death. Poliovirus has no cure once it infects the nervous system, so vaccination before exposure is the entire strategy. Thanks to global vaccination efforts, wild poliovirus now circulates in only two countries, but the disease could resurge anywhere if immunization rates drop.
What Polio Does to the Body
Most people infected with poliovirus never realize they have it. The majority experience no symptoms at all, and a smaller group develops what feels like a mild flu. But in roughly 1 out of every 200 infections, the virus reaches the spinal cord and destroys the nerve cells that control muscles. The result is sudden, permanent paralysis, most often in the legs. Among those who become paralyzed, 5 to 10% die when the virus immobilizes the muscles needed to breathe.
There is no antiviral treatment for polio. Once paralysis sets in, it cannot be reversed. And the damage doesn’t always end there. Between 15% and 80% of the estimated 20 million polio survivors worldwide develop what’s called post-polio syndrome, typically 15 to 30 years after the original illness. Symptoms include new muscle weakness, chronic fatigue, joint pain, difficulty swallowing, and breathing problems. The vaccine prevents all of this by stopping the infection before it ever reaches the nervous system.
How the Vaccine Protects You
The polio vaccine works by training your immune system to produce antibodies against poliovirus. These antibodies circulate in your blood and, if you’re ever exposed, neutralize the virus before it can travel from the gut into the central nervous system. That barrier between a routine intestinal infection and catastrophic nerve damage is entirely what vaccination provides.
Two types of polio vaccine exist, and they work differently. The inactivated vaccine (used in the United States and most high-income countries) is given as a shot. It’s extremely effective at preventing paralysis but produces a relatively weak immune response in the gut, meaning a vaccinated person could still briefly carry and shed the virus. The oral vaccine, used in many lower-income countries, triggers stronger intestinal immunity, which makes it better at stopping person-to-person transmission. This is why the oral version has been the workhorse of global eradication campaigns: it doesn’t just protect individuals, it cuts the chain of spread through entire communities.
Why Vaccination Rates Need to Stay High
Poliovirus spreads easily through contaminated water and the fecal-oral route, which means it thrives wherever sanitation is imperfect. To stop the virus from circulating in a population, vaccination coverage needs to be remarkably high. Estimates for the herd immunity threshold range from about 75% in wealthy, high-hygiene settings to as high as 97% in lower-income populations. When coverage dips below these levels, even a single imported case can spark an outbreak.
This isn’t theoretical. In 2022, poliovirus was detected in wastewater in New York and London, cities that hadn’t seen polio in decades. The virus found pockets of under-vaccinated people and began circulating silently. These events are a reminder that polio isn’t a historical curiosity. It is one plane flight away from any community with gaps in immunization.
The Scale of What Vaccination Has Achieved
In 1952, the United States recorded 20,000 cases of paralytic polio in a single year. Children were confined to iron lungs, public swimming pools closed every summer, and parents lived in fear of an invisible threat. After Jonas Salk’s inactivated vaccine was licensed in 1955, case numbers plummeted. Within a generation, paralytic polio was essentially eliminated from the country.
Globally, the numbers are staggering. Polio vaccination programs are projected to prevent 4 million cases of paralysis and 855,000 deaths over the period from 1970 to 2050. The economic returns are equally dramatic: the global program is expected to save more than $128 billion in medical care costs, far exceeding the roughly $67 billion invested in vaccination and eradication. Today, wild poliovirus persists in only Pakistan and Afghanistan, down from 125 endemic countries in 1988.
What the Vaccine Schedule Looks Like
In the United States, children receive four doses of the inactivated polio vaccine. The first dose is given at 2 months of age, the second at 4 months, the third between 6 and 18 months, and a booster between ages 4 and 6. The minimum age for the first dose is 6 weeks, and the final dose must come at least 6 months after the previous one. Most adults vaccinated as children are protected for life, though those traveling to areas where polio still circulates may need a one-time booster.
Side Effects Are Mild
The inactivated vaccine used in the U.S. has an excellent safety record. The most common side effects are soreness at the injection site and low-grade fever, both of which resolve within a day or two. The oral vaccine, still used in parts of the world, carries a very small risk of vaccine-associated paralysis: roughly 1 case per 1.4 to 3.4 million first doses. This is why most countries that can afford the inactivated version have switched to it. In studies of the oral vaccine, the most frequently reported reactions were headache (22%), stomach pain (17%), fever (12%), and diarrhea (10%), all of which were temporary. Serious adverse events were rare.
What Happens If We Stop Vaccinating
Poliovirus hasn’t been eliminated from the planet. As long as it circulates anywhere, it can reach everywhere. The virus is extraordinarily stable in the environment and spreads silently, since the vast majority of infected people show no symptoms while still shedding virus. A single unvaccinated traveler can reintroduce it to a community that hasn’t seen a case in decades.
Modeling studies have shown that if vaccination were discontinued prematurely, polio would rebound within a few years, potentially paralyzing hundreds of thousands of children annually, just as it did before vaccines existed. The investment in vaccination only pays off if the world finishes the job. Until the virus is fully eradicated, every child who misses the vaccine series represents both a personal risk and a gap in the collective shield that keeps polio from coming back.