In rare cases, the live virus in oral polio vaccine can mutate into a form capable of sparking new outbreaks. Nine African countries are currently battling polio epidemics linked to the vaccine as WHO and partners struggle to keep their efforts to eradicate polio on track. Elsewhere, cases have been reported in China, Myanmar and the Philippines.
On Thursday, WHO and partners are expected to announce they have rid the world of type 3 polio virus. There are three types of polio viruses. Type 2 was eliminated years ago. That now leaves only type 1. But that refers only to polio viruses in the wild. Type 2 viruses continue to cause problems since they are still contained in the oral polio vaccine and occasionally evolve into new strains responsible for some vaccine-derived outbreaks.
The global effort to eradicate polio was launched in 1988 and originally aimed to wipe out the potentially fatal disease by 2000. While cases have dropped dramatically, the virus remains stubbornly entrenched in parts of Pakistan and neighboring Afghanistan. This year there have been 72 cases of polio in Pakistan after only eight in 2018.
In meeting notes from an expert polio oversight board in September, WHO's Michel Zaffran said the status of eradication was "of great concern," noting the Taliban's ban on house-to-house vaccination in Afghanistan. Officials described the program in Pakistan as a "failing trajectory."