For more than 30 years, global health officials have worked to wipe out polio. But recent outbreaks have authorities rethinking vaccination strategies—since one method may fuel the problem.

Poliomyelitis was once one of the most-feared diseases in the world. The disease attacks the covering (myelin sheath) on nerves. Annual U.S. outbreaks caused thousands of cases of paralysis, mostly in children.

Outbreaks today can begin when vaccinated people shed live virus from an oral vaccine in their feces. From there, the virus can spread. This happens mostly in places with poor sanitation and low vaccination levels.

Vaccines first became available in 1955. By the 1970s, the annual number of U.S. cases was fewer than 10. Oral polio vaccine drops have been around since 1961. They’re cheap, easy to dispense—two drops directly into the mouth—and better at protecting from polio spread.

However, the drops contain a weakened form of the virus and in rare cases can actually cause polio. This uncommon phenomenon happens in about two to four children per two million doses.

Since 2017, there have been nearly 3,000 cases of polio worldwide. Only 396 have been caused by the naturally occurring (“wild”) virus. The rest came from the vaccine itself.

“We are basically replacing the wild virus with the virus in the vaccine,” says researcher Scott Barrett.

Vaccine-connected polio is even cropping up in wealthy countries, mostly in unvaccinated children.

Earlier this year, officials in Israel detected polio in an unvaccinated three-year-old, who suffered paralysis as a result. Several other children, nearly all unvaccinated, also had the virus.

This summer, British authorities found evidence in sewage that the virus was spreading, though they detected no infections in people. Still, experts say the virus poses a risk to the unvaccinated.

The virus has also shown up in New York sewers. An unvaccinated young adult there suffered leg paralysis after contracting the disease.

Genetic analyses show that the viruses in all three countries were “vaccine-derived,” or mutated from the oral vaccine.

Decades ago, many countries switched to injectable vaccines that use a deactivated fragment of the virus to prompt an immune response. The goal is to move the entire world to these shots once wild polio is eradicated. But some scientists argue the switch should happen sooner.

“We probably could never have gotten on top of polio in the developing world without the [oral polio vaccine]. But this is the price we’re now paying,” says vaccine specialist Paul Offit. “The only way we are going to eliminate polio is to eliminate the use of the oral vaccine.”

Why? Apart from Jesus, illness and death are scary and inexplicable. But God says that He works all things for good of those He loves (Romans 8:28) and most importantly, for His own glory. (John 11:4)

Note: The oral vaccine dramatically reduced the number of children paralyzed by polio. When the global eradication effort began in 1988, there were about 350,000 cases of wild polio annually. So far this year, there have been 19 cases of wild polio, all in Pakistan, Afghanistan, and Mozambique. (Read Polio in Pakistan.)