When it comes to fighting a virus, having as many weapons as possible, especially in the form of vaccines that can prevent infection, is certainly welcome. And that’s always been the case with polio, which has not one but two effective immunizations that can stop the virus from causing debilitating paralysis. Which is more effective in preventing illness and which is better at stopping transmission of the virus? Scientists report in the journal Science that neither is ideal, but that together, the vaccines are powerful enough to achieve both results. The results “revolutionize our thinking about how to use polio vaccines optimally,” says Hamid Jafari, director of polio operations and research at the WHO, who led the research.
Recent efforts to erradicate polio has pitched the two vaccines against each other. Developed in the 1950s and 1960s, one was made by Jonas Salk using killed polio virus, and the other, developed by Albert Sabin, uses a weakened but still live virus that could replicate in the human gut to deliver immunity. Jafari and his colleagues, report that children vaccinated with the oral polio vaccine who then received a boost of the Salk vaccine showed the lowest amount of virus in their feces—one of the primary ways that the virus spreads from person to person—and excreted these viruses for a shorter period of time than children who had been immunized with the oral vaccine and received a boost with an additional dose of the same oral vaccine.
The WHO’s global effort to eradicate polio has relied heavily on the oral vaccine, because it’s a liquid that can be eaily given to children orally, and it’s cheaper. Plus, the oral vaccine, because it contains a weakened virus that can reproduce in the human gut, helped to reduce the volume of virus excreted in the feces, and thus lower the chances that others coming in contact with the feces could get infected.
But in places where polio infections were rampant, such as northern India, the oral vaccines didn’t seem to be doing much good at reducing the burden of disease. Even when children were getting the recommended three doses, rates of infections remained high. “The transmission pressure was extremely high in these areas that were densely populated, had a high birth rate, poor sanitation and high rates of diarrhea,” says Jafari. In those regions, it took an additional 10 to 12 vaccination campaigns—about one a month to provide children with additional doses on top of the recommended three doses—to finally control the disease and limit spread of the virus. It turns out that the immunity provided by the oral vaccine wanes over time.
In order to eradicate the disease, public health officials knew they had to do better. So they tested whether adding in the inactivated vaccine would help. And among 954 infants and children aged five years to 10 years who had already received several doses of oral vaccine, adding a shot of the inactivated vaccine did help them to shed less virus compared to those who received another dose of the oral vaccine.
With polio currently endemic in Pakistan, Cameroon, Equatorial Guinea, and the Syrian Arab Republic, the WHO declared the spread of polio a public health emergency of international concern, and issued temporary recommendations for all residents and long-term visitors to those countries to receive a dose of either the oral or inactivated vaccine before traveling out of the country. In other countries where polio has been found, such as in some sewage samples and fecal samples from residents in Israel, health officials have also advised residents living in those regions to receive a dose of inactivated polio vaccine in order to limit spread of the virus.
“The inactivated polio virus vaccine is becoming an important tool in preventing international spread of polio,” says Jafari. Whenever outbreaks of the disease occur, health officials are now recommending that even vaccinated individuals who could be infected but not get sick, receive an additional shot of the inactivated vaccine in order to limit the amount of virus they shed and spread to others.