Drop shot: The oral polio vaccine leaves the door open for future outbreaks. Credit: Kamal Kishore/Reuters

Five children in a secluded Amish village in the US contracted vaccine-derived polio, scientists announced in October, baffling epidemiologists about the infection's origin. But some experts say the episode is a clear warning that the global polio eradication campaign is unrealistic.

As the World Health Organization (WHO) and others focus on preventing wild polio, they say, virus shed by vaccinated individuals could continue to circulate in the population.

These viruses are circulating much more extensively than anyone realizes. Vincent Racaniello, Columbia University

An eight-month-old baby in the village in central Minnesota, initially hospitalized with diarrhea and known to be immunocompromised, first contracted and transmitted the virus. But the virus itself appears to be from an oral polio vaccine administered two years ago, more than a year before the infected child was born, says Olen Kew, a virologist at the US Centers for Disease Control and Prevention.

That suggests that the virus came from someone else, perhaps in a developing country without adequate therapies to treat an immune disorder. “Someone else is exposed; it seems unlikely that that virus would have been circulating that long in the Amish community,” says Kew.

Most people who contract polio can rid themselves of infection within eight weeks, but immunocompromised individuals can retain and spread the infection, in some cases for years. Nearly 30 chronic carriers have been reported since 1962, but the Minnesota child is the first known to put a virus into circulation. She transmitted the virus to the other children because they were not vaccinated, in accord with Amish belief.

Critics say the event points to a much deeper problem in the WHO's eradication scheme. “Now we have here, in a country where almost everyone is immunized, these viruses are circulating. And they're circulating much more extensively than anyone realizes, I would guess,” says Vincent Racaniello, a Columbia University microbiologist.

Since the Americas were declared polio free in 1994, surveillance has slipped, making it difficult to assess how much poliovirus is circulating in the population. The US stopped vaccinating children with the oral polio vaccine in 2000, switching to an inactivated vaccine that confers immunity in the bloodstream, but not in the intestines or throat. Those immunized with this vaccine could potentially pick up the live, weakened strains excreted by children who receive the oral vaccine and function as carriers of the virus.

In those vaccinated with the oral vaccine— particularly in immunocompromised individuals—the virus can also mutate and recover its ability to cause paralysis. Infected feces could then trigger an outbreak in areas with low vaccination coverage. Since 2001, there have been 12 such out-breaks, costing the WHO nearly $100 million.

The WHO's plan to eradicate polio relies heavily on the oral vaccine, in part because it is cheap and easy to administer in resource-poor countries. The plan calls for all countries to simultaneously stop using the oral vaccine three years after wild polio transmission has been interrupted worldwide. But that goal has proven elusive: the deadline has been moved back three times since 2000.

Even if the WHO reaches its goal, a small number of undetected but immunocompromised individuals could silently incubate and spread polio, leaving the door open to subsequent outbreaks, experts note.

“If you have an immunodeficient person today and we stop vaccination tomorrow, then 15 years from now, everybody younger than 15 years old would be unvaccinated, and they would be unprotected,” concedes Harry Hull, Minnesota's state epidemiologist, who helped design the WHO's blueprint for eradication.

Experts say that before the WHO reaches its goal, it will need to stockpile and safeguard vaccine and seed stocks of the virus, strengthen surveillance, protect against the accidental or intentional release of polio into the environment, identify chronic carriers and develop antiviral drugs that could rid their bodies of infection.

“We do know that an unplanned strategy would be catastrophic,” says Kew says. “We've got some hard, hard work ahead.”