People often ask why we vaccinate against diseases no longer in the United States. A frequent example is polio because it has been 40 years since the last case of wild polio here. Wild polio is only seen in Afghanistan, Pakistan and Nigeria.

Polio spreads easily through contact with feces. Less commonly, it spreads by coughing and sneezing. There’s no cure for polio. Global cases have decreased by over 99 percent since 1988, from 350,000 annually to just 33 reported cases in 2018.

Recently, two cases of paralytic polio were identified in the Philippines. That may not sound alarming, but it’s just the tip of the iceberg. Over 70 percent of polio-infected people have no symptoms.

Moreover, of those that become ill, most suffer symptoms indistinguishable from those of a common cold including tiredness, fever, headache, sore throat and nausea. Paralysis only occurs in 1 out of every 200 infections.

Experts believe the Filipino outbreak is widespread because there’s inadequate surveillance to identify other cases of paralysis, the known cases are geographically far apart, and polio has been detected in sewage and waterways.

The sad thing is that the outbreak in the Philippines isn’t because of wild polio but rather a vaccine-derived strain. There are two types of vaccines, injected and oral. The injected vaccine contains killed virus while the oral vaccine contains live virus.

Live virus vaccines are usually unable to cause disease in healthy individuals. Unfortunately, the live vaccine strains may mutate and regain the ability to cause illness. This most often occurs when vaccine virus spreads from recently vaccinated individuals to unvaccinated people.

If enough of the population is unvaccinated, the vaccine strain circulates for a prolonged period, giving it time to mutate. With a third of the population unvaccinated, this is what happened in the Philippines.

In 2000, the U.S. quit using the oral (live) vaccine in favor of the injected (killed) vaccine because of the risk of mutation. Many developing countries continue to use the oral vaccine, as it’s much cheaper and easier to administer than the injected vaccine.

Infants start their polio vaccine series at around 2 months. Adults should have finished the series as a child. If they didn’t finish, they need to get their final doses. If never vaccinated, adults need to get their second dose 1 to 2 months after the first dose, and a third dose 6 to 12 months after the second. Adults traveling to countries with polio may want to get a booster dose, especially if working with people who may have polio.

Why should non-travelers care about vaccine preventable diseases abroad? These diseases can rapidly spread around the world via modern transportation. In 2014, nearly 400 cases of measles occurred in Ohio when two unvaccinated Amish missionaries carrying the illness returned from the Philippines.

Someday, polio will be eliminated from the planet like smallpox. Until that happens, we need to keep vaccinating.

Vaccine Smarts is written by Sealy Institute for Vaccine Sciences faculty members Drs. Megan Berman, an associate professor of internal medicine, and Richard Rupp, a professor of pediatrics at the University of Texas Medical Branch. For questions about vaccines, email

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