Polio Vaccines: Oral versus Inactivated

By Charlotte A. Moser, Vaccine Education Center at Children’s Hospital of Philadelphia

With recent discussions about how safe and effective polio vaccines are, some questions have come up related to the differences between the two versions of polio vaccine, inactivated and oral.

A brief history of polio vaccine use in the U.S.

The first polio vaccine was Salk’s inactivated vaccine (IPV), licensed to much fanfare in 1955. Sabin’s oral polio (OPV) was introduced as three monovalent vaccines in the U.S. between 1961 and 1962 before being replaced by a single vaccine containing all three serotypes in 1963. OPV was used routinely for the prevention of polio in the U.S. through the mid-1990s. In 1997, the CDC’s Advisory Committee on Immunization Practices (ACIP) recommended a sequential schedule of two doses of IPV followed by two doses of OPV for infants. By 2000, an IPV-only schedule was recommended and remains the recommendation in early 2025.

The reasoning behind historical changes to the polio vaccine schedule relate directly to the differences between the vaccines - as do some of the recently emerging questions - so let’s take a look. 

The pros and cons of IPV.

Inactivated polio vaccine does not reproduce. Given as a shot, the primary immune response occurs near the injection site. After licensure of IPV in the U.S. in the mid-1950s, cases of polio dropped by 95%, even in the absence of complete vaccine coverage in the population. But, when infections occurred, inevitably some of them were in vaccinated individuals. Why?

Polio virus replicates in cells that line our oral and gastrointestinal tracts and in cells of our nervous system. When someone gets vaccinated with IPV, they are unlikely to have a case of polio resulting in paralysis because the virus is likely to be stopped before having an opportunity to reach their nerve cells. However, IPV-vaccinated individuals may still experience an infection in the cells of their oral and gastrointestinal tracts before immunologic memory stops the infection. As such, if infected, these individuals could potentially shed the virus and, therefore, spread it.

In the mid-1950s, these infections in vaccinated individuals decreased confidence in IPV, so the U.S. turned to OPV.

The pros and cons of OPV.

OPV contains live, weakened polio virus that reproduces at the intestinal surface, so it generates immunity in cells of the oral and gastrointestinal tracts. This not only decreases the chance for infection of oral and intestinal cells, but it also removes the opportunity for polio virus to ever reach nerve cells. As a result, people vaccinated with OPV are unlikely to be paralyzed or even infected.

OPV also offers other advantages. As with other oral vaccines, OPV is less expensive and easier to administer. In addition, because the vaccine virus reproduces in the oral and gastrointestinal tracts, recipients shed vaccine virus, causing exposure to the virus and generation of immunity among some unvaccinated individuals. This is known as contact immunity and is particularly useful in increasing population-level immunity in areas where vaccine coverage is more difficult to achieve, such as in remote locations or areas experiencing social unrest.

While these criteria may make it seem like OPV is a better choice among polio vaccines, its use comes with one very rare, but very real, and tragic consequence - on occasion the vaccine virus reverts to wild-type polio virus and, itself, leads to cases of polio (est. 1 in every 2.3 million doses). Called vaccine-associated paralytic polio (VAPP), this condition could occur in vaccine recipients or in secondary contacts, that is people exposed to vaccine virus that was shed by vaccinated individuals.

By the mid-1990s, widespread vaccination had eliminated wild-type polio from the U.S., so the only cases of polio were in individuals affected by VAPP. Given this, U.S. policy was again changed - this time to rely on IPV, thereby preventing the remaining cases of polio occurring in the U.S. at that time.

What does this mean for population immunity?

Given that IPV does not provide as much protection as OPV, recent discussions about polio vaccine have led some to wonder how robust population immunity is in 2025. Three points are important to consider:

• Most adults in this country got oral polio vaccine for all or at least a couple of doses. The move away from OPV was finalized with the all-IPV recommendation in 2000, so as long as they were vaccinated, anyone born before late 1999 or early 2000 is likely to have protection at the intestinal surface. As such, they will be unlikely to be infected with the polio virus.

• Those born in 2000 or later who only received inactivated polio vaccine will still have immunity against polio, so even if exposed, they would be unlikely to experience severe outcomes like paralysis.

• If IPV-only vaccinated people experience a polio infection, they may shed the virus albeit at lower levels and for fewer days than an unvaccinated individual. In this scenario, it is possible that they could spread the virus to others, including to unvaccinated individuals. However, given the high levels of population immunity, cases would be anticipated to be limited.

Our evidence for this is the history of polio detection in the U.S. since its elimination in 1979. We know people infected with the polio virus enter the U.S. as a result of international travel. However, since 1979, community spread of polio has only been detected twice - recently in New York (2022) and in Minnesota in 2005. While polio was also detected in individuals in 2008 and 2013, the virus did not spread to community members.

Of course, prevention only works if people get vaccinated, so if large numbers of people stop vaccinating against polio, we could again witness the ravages of polio firsthand in the U.S. Let’s hope we don’t get that chance.

People living in other countries should consider which vaccine is used and the relative vaccine coverage rates to determine the likelihood of experiencing polio outbreaks in other places throughout the world.

Charlotte A. Moser, MS, is the co-director of the Vaccine Education Center and creator of the Parents PACK program at the Children's Hospital of Philadelphia.