The polio vaccine is an immunization that protects against poliovirus, a highly contagious virus that can invade the nervous system and cause permanent paralysis within hours. Two types exist: an inactivated version given by injection and a live oral version given as drops in the mouth. In the United States, only the injectable form has been used since 2000.
Two Types of Polio Vaccine
The inactivated poliovirus vaccine (IPV) contains killed virus. It’s given as a shot in the arm or leg and works by prompting your immune system to produce antibodies against all three types of poliovirus. Because the virus in IPV is dead, it cannot cause infection. This is the form used in the U.S. and most high-income countries.
The oral poliovirus vaccine (OPV) uses a weakened, live form of the virus delivered as drops in the mouth. OPV was the dominant vaccine worldwide for decades because it’s cheap, easy to administer without needles, and provides strong immunity in the gut, which helps stop the virus from spreading through communities. Many low- and middle-income countries still use OPV in mass vaccination campaigns.
The tradeoff with OPV is that the weakened virus can, in rare cases, mutate back into a form capable of causing paralysis. This happens when the vaccine strain circulates for a long time in communities where too few children are immunized. These outbreaks of vaccine-derived poliovirus are the reason the U.S. switched exclusively to IPV in 2000.
How the Vaccine Builds Immunity
When you receive IPV, your immune system recognizes the inactivated virus particles as foreign and begins producing antibodies tailored to neutralize poliovirus. This response is primarily antibody-based rather than the broader immune activation you’d get from a natural infection or a live vaccine. That’s why multiple doses are needed to build strong, lasting protection.
After two doses, at least 90% of recipients develop immunity to all three poliovirus types. After three doses, that number rises to at least 99%. A fourth dose is given to ensure durable, long-term protection through childhood and beyond.
The Recommended Schedule
Children in the U.S. receive four doses of IPV at these ages:
- First dose: 2 months
- Second dose: 4 months
- Third dose: 6 through 18 months
- Fourth dose: 4 through 6 years
Most adults who grew up in the U.S. were vaccinated as children and don’t need additional doses. The exception is travelers heading to areas where poliovirus is still circulating. If you’ve completed your childhood series but never received an adult booster, you may be recommended a single lifetime booster dose before travel to those regions.
Side Effects
IPV is one of the milder vaccines in terms of side effects. The most common reactions are soreness, redness, or swelling at the injection site. Some children develop a low-grade fever. Serious allergic reactions are extremely rare. Because IPV contains only killed virus, there is zero risk of developing polio from the shot itself.
How the Vaccine Changed History
Jonas Salk developed the first successful polio vaccine in the early 1950s, testing it on himself and his family in 1953 before launching a massive trial involving 1.6 million children across Canada, Finland, and the United States. His inactivated vaccine was licensed on April 12, 1955, the same day the trial results were announced.
Albert Sabin then developed the oral version. After large-scale trials on tens of millions of children in the Soviet Union and Czechoslovakia in the late 1950s proved it safe and effective, OPV became the backbone of global eradication efforts. Czechoslovakia became the first country to eliminate polio, doing so by early 1960.
Before vaccines existed, polio epidemics paralyzed tens of thousands of children each year. By 1988, when the Global Polio Eradication Initiative launched, the virus was endemic in 125 countries. Today, wild poliovirus type 1 remains confined to just two countries: Afghanistan and Pakistan. Types 2 and 3 have been eradicated entirely from the wild.
Why Polio Vaccination Still Matters
It’s easy to assume polio is gone, but the virus still circulates. As of mid-2025, Afghanistan and Pakistan continue to report new cases of wild poliovirus. Beyond those two countries, vaccine-derived outbreaks can emerge wherever immunization rates drop too low, allowing weakened vaccine strains from OPV to revert and spread.
Polio has no cure. Once paralysis occurs, it’s permanent. The only tool for prevention is vaccination, which is why maintaining high immunization coverage matters even in countries that haven’t seen a case in decades. A single infected traveler can reintroduce the virus to any under-vaccinated community.