Polio: The Disease & Vaccines
The inactivated polio vaccine (or IPV) is the only vaccine given to prevent polio in the U.S. IPV is given as a series of four shots at 2 months, 4 months, 6 to 18 months, and again at 4 to 6 years of age.
The face of polio
President Franklin Delano Roosevelt was paralyzed by polio. He was confined to a wheelchair for much of his adult life. President Roosevelt got polio when he was in his late 30s. Most of the pictures of President do not show him in his wheelchair though because he did not want the focus to be on his disability.
Only about 1 in 10 people paralyzed by polio recovered. Most were confined to wheelchairs or iron lungs for the rest of their lives. An iron lung is a negative-pressure ventilator used to help people breathe.
The disease
What is polio?
Polio is caused by a virus. Three types of polio virus exist in nature. They are called wildtype 1, wildtype 2 and wildtype 3. Wildtype 2 was eradicated in 1999, and wildtype 3 was eradicated in 2020. Eradication means the virus no longer infects anyone anywhere in the world. Wildtype polio has been eradicated in all except two countries: Pakistan and Afghanistan. Unfortunately, some of the weakened viruses used in OPV mutated to become more infectious. These are called vaccine-derived polio viruses (VDPV). VDPV still cause outbreaks in some countries (see more in the section of this page, “If polio was eradicated from all but two countries, why are there still cases in other countries?”).
Polio virus first replicates, or reproduces, in the intestines. Then it travels through the bloodstream where it can infect the brain and spinal cord. Polio virus causes paralysis when it replicates in and attacks the nervous system.
Polio virus is highly contagious and causes the same symptoms regardless of type. Polio affects people differently:
- Some don't develop any symptoms. This occurs in about 72 of every 100 people.
- Others have minor symptoms like sore throat, fever, tiredness, stomach pain, vomiting or constipation. This occurs in about 24 of every 100 people.
- A small number develop meningitis, which is inflammation of the lining of the brain and spinal cord. When caused by polio, this is called nonparalytic polio. These individuals start with similar symptoms to those with mild illness. But one or two days later, they develop a stiff neck or back, vomiting, headache and pain. Illness lasts about two to 10 days, but these people typically recover completely. This version of polio occurs in about 4 of every 100 people.
- Paralytic polio causes paralysis, muscle aches and spasms. Rates vary based on the type of polio causing the infection. Polio type 1 virus paralyzes about 1 in 200 people. Polio type 2 virus paralyzes about 1 in 2,000 people, and type 3 polio virus paralyzes about 1 in 1,000 people.
As those who survived polio infections as children have become older, another aspect of polio virus has become apparent. Some polio survivors experience symptoms similar to polio 15 to 40 years after their original recovery. The most common symptoms are muscle weakness and tiredness. This condition is known as post-polio syndrome. It’s estimated that about 25 to 40 of 100 polio survivors experience post-polio syndrome. Scientists are working to better understand what causes post-polio syndrome. Most of the evidence to date converges on roles for aging and long-term effects of the initial infection on the nervous system. Specifically, nerve cells that survived and compensated for the loss of neighboring nerve cells during the initial infection weaken or die as the polio survivor ages.
Read personal stories about polio
The vaccine
The inactivated polio vaccine (IPV) became available in 1955. It is given as a shot. A second vaccine became available in 1961. It is called the oral polio vaccine (OPV) because it is given as liquid drops in the mouth. Because it doesn’t require a shot, it is more convenient to administer than IPV.
The U.S. relied on OPV to prevent polio for almost 40 years from 1963 until 2000. The results were miraculous. The U.S. eliminated polio in 1979. The virus was eliminated from the Western Hemisphere in 1991. But polio still occurs in other parts of the world, so we still need to vaccinate against it.
The U.S. changed from using OPV to IPV in 2000. By that time, the only cases of paralytic polio in the U.S. each year were caused by the OPV vaccine. See more in the section of this page “Why do we use the polio shot (IPV) and not the oral polio vaccine (OPV)?”
How are polio vaccines made?
OPV (no longer used in the U.S.) is made using a live, "weakened" form of the three types of natural, (or wildtype) polio virus. The viruses are "weakened" by growing them in cells that are different from the ones they infect in people. This process is called "cell-culture adaptation" (see How Are Vaccines Made?). "Cell-culture adaptation" alters natural polio virus, so it does not infect our cells as efficiently when it is given in the vaccine. Polio virus grows in human cells in the intestine, brain and spinal cord. The vaccine viruses are weakened by growing them in monkey kidney cells. OPV is given by mouth, so the virus replicates in the intestines. This causes antibodies to be made in the intestines. If a person is exposed to the virus, these intestinal antibodies stop the infection from progressing to the bloodstream where it can cause paralysis. Because the vaccine virus replicates in the intestines, people also shed the vaccine virus in their feces after vaccination. This can cause the virus to spread to other people. If those people are not already immune, they may also develop immunity from exposure to the vaccine virus. This is called contact immunity.
IPV is made by killing polio virus with a chemical (formaldehyde). The dead virus cannot reproduce itself so, it cannot possibly revert back to natural polio or cause paralysis. IPV is given as a shot, so the antibodies from vaccination circulate in the bloodstream. Antibodies are not present at high levels in the intestines (like those produced after OPV). The result is that if someone is exposed to natural (or wildtype) polio, the virus often still reproduces in the intestines. The virus is stopped when it gets to the bloodstream. This prevents it from reproducing in the brain or spinal cord, so the person will not be paralyzed. But because the virus replicates in the intestine, people can still spread polio virus to others.
What are the side effects of the polio vaccine?
OPV (no longer used in the U.S.) causes immunity by delivering a live, weakened form of polio virus. But sometimes the vaccine virus changes back to the original wildtype form during replication. When this happens, it can cause a person to develop paralytic polio. This is known as vaccine-associated paralytic polio or VAPP. It happens in about 1 of every 2.4 million doses. Most often, vaccine recipients experience VAPP. But in rare cases, VAPP occurs in susceptible people exposed to vaccine virus shed by vaccine recipients.
IPV causes redness and pain at the injection site and fever in some recipients. About 1 in 1 million recipients can have a serious allergic reaction. If the allergic reaction occurs, it will happen in the first 15-30 minutes after vaccination. Parents are encouraged to wait at the vaccination location for that period of time after any vaccines because the reaction can be medically treated.
Other questions you might have
Why do we use the polio shot (IPV) and not the oral polio vaccine (OPV)?
About 1 of every 2.4 million doses of the oral polio vaccine (OPV) causes paralysis. This happens when the virus in OPV changes back to wildtype virus. The condition is called vaccine-associated paralytic polio (VAPP). It can happen to vaccine recipients or to susceptible people exposed to vaccine virus shed by vaccine recipients.
The shot (IPV) cannot cause VAPP because it contains killed virus. The killed virus cannot replicate and, therefore, cannot cause paralysis.
Between 1961 and 1996, children in the U.S. received four doses of OPV. Wildtype polio was eliminated from the U.S. in 1979, so throughout the 1980s and 1990s, the only people paralyzed by polio in the U.S. were those who developed VAPP. The U.S. changed to using only four doses of IPV by 2000.
Many other countries continue to use OPV because it is more economical and easier to administer. This enables more people to get the polio vaccine. OPV also provides better community immunity.
Find out about the history of polio vaccine development in “A Closer Look – The Fascinating Science of Polio Vaccines” from The Hilleman Chronicle newsletter.
Do adults need polio vaccine?
Adults 18 years and older known to be unvaccinated or partially vaccinated against polio are recommended to get polio vaccine. If their vaccination history is unclear, they should get three doses. Dose two should be given four weeks after the first dose. Dose three should be given six to 12 months after dose two.
Previously vaccinated adults at increased risk for exposure to polio can get one booster dose.
If polio was eradicated from all but two countries, why are there still cases in other countries?
OPV uses live, weakened polio virus that reproduces in the intestine of vaccine recipients. Two benefits of this approach relate to transmission of the virus. First, when the vaccine virus is shed by vaccine recipients, it is most often the weakened form. Others can gain immunity if they are exposed to the vaccine virus in this way (called contact immunity). Second, when a vaccinated person is exposed to natural (wildtype) polio virus, the virus is stopped before it can replicate in the intestines and spread to others.
But in rare instance, the live, weakened vaccine virus in OPV can revert back to the natural or wildtype version. When this happens, it can cause vaccine-associated paralytic polio (VAPP), and the virus can continue to spread. These types of polio virus are called “vaccine-derived polio virus” (VDPV).
OPV was an important tool for moving toward eradication, which is the complete elimination of polio throughout the world. But as we get close to the finish line, often the remaining cases are those caused by VPDV. It is these viruses that cause outbreaks in countries that have eliminated natural (wildtype) polio.
The Global Polio Eradication Initiative (GPEI) tracks cases of polio throughout the world each week. You can check the summary of weekly cases on their website.
I heard about a case of polio in NY in 2022. Why did it occur and what does it mean?
Polio was eliminated from the U.S. in 1979. But on a few occasions, cases have been identified in the U.S. During the fall of 2022, an unvaccinated man in Rockland County, New York (NY) was paralyzed by a polio infection. The NY man was infected with a type of polio virus that came from OPV (See “If polio was eradicated from all but two countries, why are there still cases in other countries?”). This is called vaccine-associated paralytic polio or VAPP.
The NY case was concerning because only about 1 of every 2,000 people infected with this type of polio will be paralyzed. That means when we saw a case in the U.S. it was likely that many others near where the man lived had also been exposed to the virus. High vaccination rates in most U.S. communities mean that often we remain unaware of the presence of polio virus. But if immunizations rates drop, more cases could occur.
Dr. Offit discussed the 2022 case and what it could mean in this video. He also discussed the history of polio and polio vaccines.
Relative risks and benefits
Do the benefits of the inactivated polio vaccine (IPV) outweigh its risks?
Natural, or wildtype, polio was eliminated from the United States in 1979. However, polio has not been eliminated from the rest of the world. Efforts to eradicate polio have proven challenging. Polio has never been successfully eliminated from Pakistan or Afghanistan. We also see cases of polio in other places because of use of OPV vaccine. So, all countries remain at risk for the return of polio. Most people exposed to polio will recover without long-term effects, but some will be paralyzed, and some will die.
The vaccine used in the U.S. is IPV. IPV has no serious side effects, so the benefits of the vaccine clearly outweigh its risks.
Disease risks
- Sore throat, fever, tiredness, stomach pain, vomiting or constipation
- Meningitis
- Muscle aches, spasms or permanent paralysis
- Death
Vaccine risks
- Pain, redness and swelling at the injection site
- Fever
- Rare allergic reaction (about 1 in 1 million doses)
Resources
Reference
Orenstein W, Offit PA, Edwards KM and Plotkin SA. Poliovirus vaccine-inactivated and poliovirus vaccine-live in Plotkin’s Vaccines, 8th Edition, 2024, 890-968.
Reviewed by Paul A. Offit, MD, on May 19, 2025