Edited live vaccine could stop harmful polio outbreaks

We’re on the brink of wiping out polio, but the virus used in vaccines keeps evolving to become harmful again. The discovery of how the virus mutates to do this could lead to a safer vaccine. 

Polio once killed hundreds of thousands of children every year. The disease has largely been brought under control by the oral polio vaccine, which contains a weakened form of the live poliovirus. We do have a vaccine that uses dead virus instead, but this is less effective at spreading immunity. When the weakened live vaccine reaches the intestine, the virus replicates and can be passed on to others in close contact, transmitting immunity to people who haven’t been vaccinated.

It’s an effective way of wiping out polio, but it carries a risk. From time to time, the weakened virus re-evolves the ability to cause disease, and spreads rapidly through unvaccinated populations. In the 10 years leading up to 2015, there were around 750 reported cases of paralysis caused by vaccine-derived poliovirus (VDPV) worldwide.

A new vaccine in development may put a stop to this. Raul Andino and colleagues at the University of California, San Francisco, have analysed the genes of 424 samples of VDPV from 30 different outbreaks, and compared them with the genetic makeup of the vaccine. They found that in every case, the weakened vaccine virus had undergone the same three evolutionary steps to become virulent again.

Andino’s team have designed a live virus vaccine that should have a lower risk of re-evolving virulence. By increasing the accuracy of how the virus replicates, they have reduced the likelihood of it acquiring the necessary mutations to become harmful again, and have also made it less able to swap genes with other viruses. “What we’re trying to do is put the virus in an evolutionary cage so it can’t evolve further,” says Andino.