Polio: The Disease & Vaccines

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. 

Watch this video, “The Iron Lung and Polio,” by Mark Rockoff, MD for OPENPediatrics to learn more about the iron lung.

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.

  • Paul Offit, MD: Hi, my name is Paul Offit. I'm talking to you today from the Vaccine Education Center at the Children's Hospital of Philadelphia. It's Friday, December 9th, 2022. Well, this summer something unusual happened in Rockland County, New York, and that is that a 27-year-old unvaccinated man was paralyzed by poliovirus.  

    Now, we eliminated poliovirus from the United States by 1979. So, what's happening? And I think to understand what happened to this man, we need to go back to the beginning, which is the mid-1950s at a time when polio would paralyze 25[,000] to 30,000 children a year, would cause 1,500 children to die every year because we didn't have a vaccine. But in the middle of the 1950s, around 1955, Jonas Salk made a polio vaccine. He made it by taking the virus, growing it up in cell culture, purifying it, and then completely inactivating it with a chemical, a so-called whole killed viral vaccine. And we used that vaccine in the United States from 1955 up until 1962.  

    in 1962, that vaccine was replaced by Albert Sabin's vaccine, which was a live-weakened form of the virus. We have other live-weakened viral vaccines, like the measles vaccine, or the mumps vaccine, or the German measles (or rubella vaccine), or one of the rotavirus vaccines, or the chickenpox vaccine. Those are all made the same way by taking a virus, in this case poliovirus, and there's three strains of poliovirus, strains 1, 2, and 3, and what Dr. Sabin did was he weakened those viruses, the natural viruses, the wildtype viruses, by growing them up in cell culture, in monkey cells. And that vaccine then was then used in the United States from the early 1960s up until around the year 2000, and it eliminated polio from the United States by 1979; it eliminated polio from the Western hemisphere by 1994. But unfortunately, there was a problem with that vaccine, and it wasn't trivial, and that was that in some people, and it was extremely rare, roughly 1 out of every 2.4 million people who got that vaccine would develop paralysis caused by the vaccine virus. Because the vaccine virus, which would reproduce itself in the intestine over and over again, could occasionally revert to essentially so-called neurovirulent strain or a wildtype strain. I mean, it really, it caused a clinical syndrome indistinguishable from polio. So, in other words, this was called vaccine-associated paralytic polio, and it was rare, but it was real.  

    So, although we eliminated polio from the United States by 1979, throughout the 1980s, throughout the 1990s, every year, eight to 10 children would be paralyzed by that vaccine. For that reason, in the year 2000, we then eliminated the use of the oral polio vaccine. Albert Sabin's oral polio, live-attenuated viral vaccine was eliminated for use in this country in 2000. So, for 20 years now, more than 20 years, we have only used the inactivated vaccine, and that's why we then eliminated even vaccine-associated paralytic polio. But there are many countries that still use the oral polio vaccine, and for that reason, there are those viruses, those revertant viruses, those vaccine-associated paralytic polio strains, still circulate. And that's what happened to this man. What happened to this man was that he was, because he was unvaccinated, that virus, that type 2 revertant virus that was originally derived from Albert Sabin's polio, Albert Sabin's live-attenuated polio vaccine, that caused him to be paralyzed. 

    Now, know this, know that only about 1 of every 2,000 people who are infected with that vaccine-derived strain will become paralyzed. So, his case of paralysis represents the tip of a much, much bigger iceberg. And you know that's true because what has happened at that point was, we looked at wastewater in Rockland County, looked at wastewater in surrounding counties, like Sullivan County, Ocean County, and found that there was this type 2 revertant virus, this vaccine-associated virus was in the wastewater, so that says that at least thousands, and possibly tens of thousands, and possibly hundreds of thousands of people could have been infected with that virus. Now, the inactivated polio vaccine, which we use in this country and have only used since the year 2000, is very good at preventing poliovirus from entering the bloodstream and traveling to the brain or spinal cord and causing harm. So, anybody who’s vaccinated with the inactivated polio vaccine will be prevented from getting polio if they are infected with this type 2 revertant strain from the vaccine. But the inactivated vaccine doesn't induce intestinal immunity. So, you could be infected with this type 2 revertant paralytic strain and shed it in your stool, and then pass it on to others. You'll never be paralyzed, and those who are vaccinated will never be paralyzed, but they can still spread the virus.  

    So, I think what we need to do, what it's important to do now, is to look in the wastewater not only in New York or in Rockland County, but in Philadelphia; in Portland, Oregon; in Las Vegas, Nevada; in Los Angeles, California; in Indianapolis, Indiana, to answer the question, how prevalent is this particular strain? This neurovirulent type 2 strain that was derived from the vaccine, which you know is circulating throughout the world and may well be circulating in this country. And if we find that that's true, it just makes it all the more important to make sure that we've gotten the inactivated vaccine, which will then prevent us from being paralyzed by this virus but won't prevent us from transmitting this virus. This is an easily solved problem, just make sure everybody who's gotten the inactivated vaccine gets it. If you're an older person like me and you got the oral polio vaccine … you got the inactivated vaccine in the '50s and then the oral vaccine in the 1960s, or if you got the oral polio vaccine from the 1960s up till the year 2000, you are protected against this virus, but you're not protected if you're not vaccinated. So, we need to make sure that everybody's vaccinated and we're going to learn, I think, over the next few months or year just how common this particular type two revertant strain is in this country.  

    Thank you. 
Transcript Transcript

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