Polio is a tricky foe. One of the biggest hurdles in the World Health Organization's polio eradication campaign is that the virus causes no symptoms in 90 percent of people who contract it. But these silently infected individuals can still spread the virus to others by coughing, sneezing or shedding it in their feces. And those they infect may become permanently paralyzed by or die.
Polio's evasiveness has also led to a big speed bump on the road to eliminate the disease. As I report in the current issue of Inside Stanford Medicine, scientists are trying to figure out how to stop a form of poliovirus that is derived from one type of polio vaccine. Oral vaccines, which consist of live poliovirus that has been inactivated, can occasionally mutate in someone's intestines to regain infectiousness. And, in rare instances, these viruses escape to the environment in feces, spreading to other people via sewage-contaminated water.
These "circulating vaccine-derived viruses" are threatening to overtake naturally occurring, "wild" poliovirus as the main source of paralysis in the communities where polio persists. The CDC's most recent report on polio infections in Nigeria says that during the first nine months of 2014, the vaccine-derived viruses caused 22 cases of paralyzing poliomyletis, whereas wild virus caused six cases, for instance.
To tackle the problem, researchers are investigating how the injected polio vaccine, which is made with killed virus, might be substituted for the oral vaccine. The injected vaccine has some potential disadvantages for use in developing countries, so it's not necessarily an easy substitution. In my story, Stanford's Yvonne Maldonado, MD, who is studying the problem with a grant from the Bill & Melinda Gates Foundation, explains:
“We don’t really understand how well the killed vaccine is going to work in kids in developing countries, where there is lots of exposure to sewage, and malnutrition leaves children with weakened immune systems,” Maldonado said.
Her Gates Foundation grant examines semi-rural communities in Mexico where children now receive routine doses of the killed vaccine, followed by twice-a-year doses of the live vaccine.
“It’s an opportunity for us to study a natural experiment,” Maldonado said. Her team wants to know if the primary immune response to the killed vaccine will reduce shedding and transmission of later doses of live vaccine. They hope that starting with one or more doses of the injected vaccine will give kids the best of both worlds: from the shot, protection against circulating vaccine-derived viruses; from the oral vaccine, intestinal immunity.
Previously: TED talk discusses the movement to eradicate polio and New dollar-a-dose vaccine cuts life-threatening rotavirus complications by half
Photo of children in South Sudan receiving oral polio vaccine by United Nations Photo