If polio is near extinction, why do outbreaks still pop up in places where the disease was thought to be long gone? The answer is complicated.
Global efforts to destroy disease-causing polioviruses have been quite a success story. Cases caused by the wild poliovirus have dropped 99% since 1988, thanks to vaccination efforts and a public-private partnership launched that year called the Global Polio Eradication Initiative.
Yet some immunization efforts carry the very rare risk of causing polioviruses to circulate in areas where many people might not yet be vaccinated or areas that were poorly vaccinated — an event that could lead to new cases of disease while trying to demolish it.
It turns out that live strains of poliovirus that are used in the oral poliovirus vaccine can mutate, spread and, in rare cases, even trigger an outbreak, representing a catch-22 in ongoing polio eradication endeavors.
These vaccine-derived viruses continue to haunt certain regions of the world.
‘We cannot stop our efforts now’
“Why do we see outbreaks? First, we need to make a distinction between the wild poliovirus — which is the virus we’re trying to eradicate — and the vaccine-derived polioviruses, which are causing some of the outbreaks that we are now seeing in the Democratic Republic of Congo, as well as in Papua New Guinea,” said Michel Zaffran, director of the polio eradication program at the World Health Organization in Geneva, Switzerland.
Last month, a polio outbreak was confirmed in Papua New Guinea, which had been certified as polio-free in 2000, along with the rest of WHO’s Western Pacific Region. Last year, two separate outbreaks of polio were reported in remote parts of the Democratic Republic of Congo. Another outbreak was confirmed in Syria.
These outbreaks all involved circulating vaccine-derived polioviruses.
Symptoms of polio, or poliomyelitis, include fever, fatigue, headache, vomiting, stiffness in the neck, pain in the limbs and, in a very small percentage of cases, paralysis, which is often permanent. There is no cure for polio, and it can be prevented only by immunization.
-derived polioviruses circulating have been rapidly stopped in the past with two to three rounds of immunization campaigns, according to WHO. So, for all polio outbreaks — from a vaccine-derived virus or not — the solution has been to immunize every child several times with the oral vaccine to stop transmission.
“The eradication of the wild poliovirus is on track because we only have three countries where the wild virus still fights for survival: Pakistan, Afghanistan and Nigeria. I believe that we could very well eradicate the virus very soon,” Zaffran said.
“In terms of these outbreaks caused by the vaccine-derived viruses, this can only be stopped when we stop altogether using the vaccine, and we will not stop using the vaccine until we have eradicated the wild virus. So it’s a chicken and egg situation,” he said. “But we first need to eradicate the wild virus. After that, we will actually withdraw globally the use of the oral polio vaccine.”
Vaccine-derived polioviruses circulate when routine immunization efforts are poorly conducted, leaving a community susceptible to any poliovirus, whether vaccine-derived or wild.
Experts tend to agree that the problem is not with the vaccine itself but rather low vaccination coverage, as a fully immunized community would be protected against both vaccine-derived and wild polioviruses, according to WHO.
“That’s why the world needs to continue its efforts until the very end,” Zaffran said. “We cannot stop our efforts now.”