What Is 'Vaccine-Derived Polio,' and How Much Should You Worry?

Polio has been nearly wiped out worldwide, but now there is a known case in the U.S. for the first time in nine years. The person is unvaccinated, but the strain they were infected with is a type of what’s called “vaccine-derived” polio. Let’s talk about what that means—because it does not mean you should avoid the polio vaccine.

What is polio?

Poliomyelitis is a virus that causes no symptoms or mild flu-like symptoms in most people who contract it, but occasionally it can attack the nervous system and cause paralysis or even death.

Two polio vaccines were developed against it: an injectable vaccine with killed virus in the 1950’s, and an oral vaccine with weakened live virus in the 1960’s. (These are often called the Salk and Sabin vaccines, respectively.) A global eradication campaign was begun in 1988, and as of 2021, there were only six known cases of wild polio infections worldwide.

Eradication means vaccinating people all over the world, with a special focus on places where polio is still circulating. Since humans are the only species that can carry polio, the idea is to make everyone immune to the disease and then it will have no hosts and will die out. Of the three types of wild polio, two have already been eradicated.

What do the vaccines do?

There are two types of polio vaccines, and they are used in different parts of the world because of the different risks and benefits of each.

The inactivated polio vaccine (IPV) is the only polio vaccine that has been used in the U.S. since 2000 and is currently one of the routine vaccines that babies get here. It’s the safest polio vaccine for the person who gets it, since the virus is inactivated and cannot make you sick. The virus contained in the vaccine cannot spread to others.

The downside is that, even when you are vaccinated, if you do catch the polio virus, you are protected from getting sick but you could still pass it on to others.

Because of these pros and cons, it makes the most sense to use this vaccine in places where polio is not commonly found. You’re personally protected from polio if a case were to make it to your neighborhood, but in a place where there are ongoing outbreaks, it makes more sense to use a vaccine that prevents transmission.

The oral polio vaccine (OPV) is used in places where polio is more common. It prevents transmission as well as infection.

It also has a bonus feature where your body can actually spread the vaccine virus to others, basically vaccinating them as well. This can happen because the virus in the vaccine is “alive” but weakened. Adding to the convenience, it’s easy to administer. You don’t need a healthcare provider who knows how to give injections; you just need somebody to squeeze a few drops of vaccine into a kid’s mouth, or even put the drops on a sugar cube to be eaten.

Since this vaccine uses a live, attenuated virus, it’s risky for people who are immunocompromised. And because it can spread, it’s also not recommended for people in their household.

There are a few other important risks to OPV, though, and these are why it’s avoided in places without endemic polio. One is that, in very rare cases, the vaccine virus can cause paralysis (called VAPP, for vaccine-derived paralytic poliomyelitis). This is estimated at one case in every 2.4 million vaccine doses given, or 0.00004%.

The other is that, again in rare cases, the vaccine virus can mutate and become able to make people sick. This is what’s called “vaccine-derived polio.” People who have been vaccinated are still protected from vaccine-derived polio, but it can spread to unvaccinated people.

How can you avoid getting vaccine-derived polio?

This one is simple: Get a polio vaccine.

Remember, people who have been vaccinated against polio are protected from it. If you get the full three-dose series of IPV, the vaccine that’s used in the U.S., effectiveness is 99% to 100%.

You are only susceptible to vaccine-derived polio if you are unvaccinated and if you come into contact with the virus. For example, if you travel to an area where OPV is in common use, you would want to make sure you get vaccinated yourself. But even without traveling, you may encounter somebody who is carrying a vaccine-derived polio strain (for example, if they traveled from a place that uses this vaccine) and they would be able to pass on the virus to you.

This is likely what happened with the case that was reported in New York. The patient had paralysis, and was found to be infected with a vaccine-derived strain of polio. They were unvaccinated. The Rockland County and New York State health departments have issued a statement advising vaccination for “individuals who are unvaccinated, including those who are pregnant, those who have not completed their polio vaccine series previously, or community members who are concerned they have might have been exposed.”