Pressing questions about polio, answered

The first polio case in the U.S. since 2013 emerged in July, and cases have increased abroad. Yvonne Maldonado answers questions about the virus and its impacts.

- By Emily Moskal

A health care worker administers a polio vaccine in Pakistan.
A M Syed/

This July, polio was detected in New York in someone who was not vaccinated against the disease. It was the first case in the United States since 2013.

While the state declared an emergency in the months after the diagnosis because the virus was detected in wastewater, experts say a big U.S. outbreak is unlikely, thanks to high vaccination rates and good hygiene. But global statistics are concerning: Pakistan reported 19 cases of polio virus circulating this year, up from only one in 2021.

According to the Centers for Disease Control and Prevention, the U.S. person infected with polio caught it from virus shed by a person who received a live vaccine administered abroad. Though the live vaccine hasn’t been used in the United States since 2000, it still is administered in many developing countries.

In the U.S., clinicians use an injected vaccine with parts of the virus that are not live but still trigger an immune response. This version is safer, but it’s more expensive to produce and problematic in developing countries because it needs cold storage and specialized training for administration. Fortunately, a new, more stable oral polio vaccine, rolled out in March of last year, could stop a bigger outbreak abroad, according to Yvonne Maldonado, MD, infectious disease expert at Stanford Medicine.

I discussed the polio outbreak, what’s causing it and how to stop it with Maldonado.

What is polio and how is it spread?

Polio is a life-threatening infectious disease caused by a virus that spreads through contact with an infected person, primarily through fecal matter.

Yvonne Maldonado

There are three strains of the polio virus, and they are a part of a family of more than 100 enteroviruses, which infect the intestine. Polio is a small RNA virus that grows in the intestinal tract of an infected person, but in rare cases, it can escape into the central nervous system and cause damage. Rarely — between 0.05% and 5% of cases — the virus can cause paralysis. The likelihood depends on the virus strain.

Between 1% and 20% of infected people will develop meningitis — a condition in which membranes covering the brain and spinal cord swell — as a result of the disease. More commonly, people with a polio infection will develop flu-like symptoms — about 25%.

Isn’t polio eradicated?

In 1988 the World Health Organization announced an effort to eradicate polio, and scientists and doctors deployed the vaccines, which were developed in the 1950s, widely. Eradication largely happened for two polio strains: Type 2 and Type 3 polio. Type 1 is still regularly circulating in some countries, primarily Afghanistan and Pakistan.Vaccine-derived poliovirus can be found in countries in sub-Saharan Africa, which still offer oral live vaccines. There have been documented cases of the live vaccine leading to immediate paralysis from directly administered live vaccine, but it is exceptionally rare. Only about 100 cases have been documented since 1960 among the hundreds of millions of vaccines administered per year.

In 1979, the U.S. was declared polio-free, and worldwide cases dwindled. In 2021, Pakistan, one of the few countries where polio still circulated, reported no cases. But this past year two dozen cases have emerged around the world.

We were on the brink of eradicating polio — we have the resources — but vaccine uptake globally is still a problem.

I’m not sure if I’m vaccinated. How can I find out? Do I need a booster?

Your pediatric provider has vaccination records. Most likely, if you are under 60 years of age, you received the three-dose series of inactivated (not live) virus as a child. If you are unsure, it’s safe — and recommended — to get the vaccines, even if you have already been vaccinated. The CDC now recommends children receive four doses of the polio vaccine.

The new oral vaccine, which was jointly developed by the CDC’s polio laboratory and other institutions, has less risk of neurological side effects and is approved for emergency vaccination campaigns in countries using live vaccines.

There is some evidence that people who had their vaccines more than 20 years ago can, in rare cases, develop paralytic polio. If you are concerned about future exposure, a booster is recommended. That will protect you against symptoms, but it’s important to remember that if you are infected, you can still shed virus and infect others, and unvaccinated people who are infected may be at risk of developing paralysis.

What can I do to protect myself?

Precautionary measures, such as washing hands before eating, can protect a person from catching polio. But the most important and effective action anyone can take is to get the inactivated vaccine. And, if you’re traveling to a country with known cases, a booster is recommended.

About Stanford Medicine

Stanford Medicine is an integrated academic health system comprising the Stanford School of Medicine and adult and pediatric health care delivery systems. Together, they harness the full potential of biomedicine through collaborative research, education and clinical care for patients. For more information, please visit

2023 ISSUE 3

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