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Polio and the Polio Vaccines

Both oral and injected poliovirus vaccines are safe and effective at protecting against polio.

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Public Health On Call
Photography by Bettman / Getty Images

The polio vaccine is a public health success so profound that few people in the U.S. even think about the disease the vaccine prevents.

But before the vaccine was approved in 1955, polio was a widely feared disease that largely affected children. Some kids infected with poliovirus died; some were paralyzed and lost the ability to walk; others whose breathing muscles were paralyzed were treated in iron lungs—or lived out their lives in the breathing machines.

That 1955 vaccine—the inactivated polio vaccine, or IPV—is the only polio vaccine used in the U.S. today. Over the past seven decades, it has proven incredibly safe and effective.

So why are doubts emerging about the vaccine’s safety and effectiveness? Two main sources fuel these concerns.

One is a petition(link is external) filed by attorney Aaron Siri to revoke the FDA approval of the polio vaccine. The petition asserts that the FDA’s approval of polio vaccines was granted without adequate testing of them. The other is confusion about whether polio vaccines can actually cause polio—what is known as vaccine-associated paralytic polio (VAPP), which is extremely rare and not associated with the IPV.

Vaccine expert William Moss, MD, director of the International Vaccine Access Center, joined host Lindsay Smith Rogers to talk about polio and polio vaccines on the January 29 episode of Public Health On Call(link is external). This article adapts the podcast interview, with Moss’s responses consolidated and condensed.

Polio and What Causes It

Polio is an infectious disease caused by polioviruses. There are three types of polioviruses: types 1, 2, and 3. The disease is transmitted through what we call the fecal-oral route: The virus goes into the mouth, into the gut, comes out the other end, and is transmitted through contaminated water or food or even hands from one person to the next.

Many people who acquire poliovirus infections, particularly early in childhood, will have a relatively minor illness. Some will have a fever or a flu-like illness. In more serious cases, they'll get what we call aseptic meningitis, which is inflammation around the brain, around the meninges.

But what we really worry about, and it occurs in less than 1% of infected people, is the paralytic form of the disease, which can lead to lifelong disability. If it affects the spinal cord high up, it can impact breathing, and it can lead to death.

All three types of the virus can cause this serious form of the disease.

The Polio Vaccines

There are two broad types of polio vaccines: the inactivated poliovirus vaccine and the oral polio vaccine.

The Inactivated Poliovirus Vaccine (IPV) 

The first polio vaccine, the inactivated poliovirus vaccine, was developed by Jonas Salk in the United States and approved in 1955. Polio was a dreaded, feared disease, and it's hard for us today to imagine the delight and celebrations that took place when the results of the trial were announced in 1955.

That vaccine is called a trivalent vaccine because it's effective against each of the three types of polioviruses. It is called an inactivated virus vaccine because it uses killed viruses. This vaccine is administered by injection.

The Oral Polio Vaccine (OPV)

In the early 1960s, Albert Sabin developed the oral polio vaccine, or OPV. It is a trivalent vaccine against all three types, but it is a different type: an attenuated virus vaccine.

Attenuated viral vaccines—which include the measles, mumps, and rubella (MMR) vaccines—use viruses that have been weakened by growing them in nonhuman cells. The viruses adapt to those cells, and when they're put back in humans, they induce an effective immune response, but typically don't cause disease.

The big advantage of the OPV is that it is administered orally: two drops of liquid in the mouth. It's very easy to deliver—it doesn't require a needle and syringe, so you don't need trained health care workers to give it.

That ease of delivery led a lot of countries, including the U.S. initially, to adopt the OPV. It became the basis of the global polio eradication effort, and that vaccine was very highly successful, leading to a 99.9% reduction in the global incidence of polio.

OPV Drawbacks—and Solutions

An important drawback of the OPV is that the attenuated polioviruses can mutate to a pathogenic form and cause vaccine-associated paralytic poliomyelitis, or VAPP. It is rare—around five cases per 1 million births, resulting in about 250 to 500 cases per year—but it is obviously a huge concern. This can happen with all three types, but type 2 in particular has a propensity to revert to that neurologically virulent form and cause paralysis.

Because of this risk, in 2000 the U.S. switched from the OPV to the IPV. The IPV cannot cause paralytic disease because it uses a killed virus that cannot mutate.

But many countries around the world continued to use the OPV, and those mutated forms of the virus began circulating. These are what we call circulating vaccine-derived polioviruses, and we did see a paralytic poliovirus case in New York in 2022(link is external) caused by one of these viruses. In that case, an unvaccinated person imported one of the circulating vaccine-derived polioviruses from another country into an Orthodox Jewish community in New York that had very low vaccine coverage against polio.

But it’s important to remember that, as I mentioned before, less than 1% of infections with poliovirus cause paralytic disease. So when you see a paralytic case, often it can mean that a lot more people are actually infected. In New York, they did identify circulating vaccine-derived polio viruses in the wastewater in a larger geographic area than that one community. A targeted campaign(link is external) followed to vaccinate susceptible children.

The Global Switch

In 2016, the World Health Organization and partners recommended “the switch(link is external),” which involved two things: (1) switching from the trivalent OPV to a bivalent OPV that had only types 1 and 3, and (2) introducing a single dose of the IPV to provide protection against all types, including type 2.

Several years ago, researchers also developed a novel oral poliovirus vaccine for type 2, or nOPV2(link is external). They made some deliberate genetic changes to the poliovirus so that it was less likely to become virulent. nOPV2 is now used to help control outbreaks of the circulating vaccine-derived poliovirus 2.

No one is using the original trivalent OPV anymore. Everyone has switched to the bivalent OPV plus the IPV, and the nOPV2 is used only in places where there are outbreaks of circulating vaccine-derived poliovirus 2.

Where We Are Now

Of the naturally occurring, or wild types, of poliovirus, types 2 and 3 have been eradicated: They are no longer circulating anywhere in the world. Type 1 of the wild-type poliovirus is circulating now only in two countries, Afghanistan and Pakistan.

In the U.S., polio has been eliminated, meaning that it is not circulating in this specific geographic area. This includes both wild type and vaccine-derived versions of the virus.

Outbreaks of circulating vaccine-derived polioviruses tend to happen in places where immunization rates against polio are low and where sanitation is poor, facilitating the fecal-oral transmission of polioviruses. These viruses are now circulating in about 45 countries around the world, particularly throughout many countries in Africa and in Southeast Asia.

But again, going back to the 1930s, ’40s, and ’50s here in the U.S., polio was a feared disease across the country, and the U.S. has good sanitation systems. Transmission is mitigated in part by having good sanitary systems, but you still need high vaccination coverage.

Vaccination Until Eradication

When used effectively, vaccines make diseases disappear. Polio is a great example. When you look at the numbers of polio cases in the U.S. over the 20th century, you see all these spikes in the first half—and then nothing in the second half. That's because of vaccines.

But we can't take that success for granted. Until a disease is eradicated, we need to continue our efforts. As with all infectious diseases, if there's an outbreak anywhere, it puts others at risk.

The IPV that's used in this country is a very, very safe and effective vaccine. It can prevent us from going back to those days where parents have to fear that their healthy child could be paralyzed the next day. To have even a single case of a child paralyzed by polio in the United States would be enormously tragic—and completely preventable.

 

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