Pakistani polio vaccination team administering polio drops.

A child receives polio drops in Pakistan, where outbreaks of the potentially paralysing disease have been traced to a vaccine no longer in use. Credit: Rana Sajid Hussain/Pacific Press/LightRocket/Getty

Vaccines

Outdated polio vaccine causes new cases of disease

Dozens of polio outbreaks in the past few years have been linked to a polio vaccine that many nations abandoned in 2016.

An old vaccine against polio is contributing to fresh outbreaks of the disease — even though the vaccine was phased out years ago.

Polio cases have fallen worldwide by nearly 100% since 1988, thanks to efforts to immunize children against the poliovirus. But the weakened viruses used to make the standard oral polio vaccine can spread from vaccinated individuals and persist in a community with many unvaccinated people. These viruses can then mutate into a form that can cause paralysis.

Hoping to address this problem, more than 150 countries switched to a new vaccine in 2016. To investigate vaccine-derived polio outbreaks since the switch, Grace Macklin at the World Health Organization in Geneva, Switzerland, and her colleagues analysed data from polio paralysis cases that occurred between May 2016 and November 2019 in several African and Asian countries.

The researchers counted 41 outbreaks, which were linked to 859 vaccine-derived polioviruses. About 65% of those viruses probably emerged after the vaccine switch.

Because vaccine-derived polio outbreaks are becoming more probable over time, a new oral vaccine that doesn’t mutate easily is needed, the researchers say.