Last week, the Global Polio Eradication Initiative (GPEI) reported seven children, six in the Democratic Republic of the Congo (DRC) and one in neighboring Burundi, had recently been paralyzed by poliovirus strains derived from a vaccine meant to prevent the disease.
Unfortunately, such cases are so common—786 were reported last year in Africa, Yemen, and elsewhere—that these seven might not have stood out against the noise. But there was a key difference, GPEI said in a 16 March statement: These are the first cases linked to a new polio vaccine that was painstakingly designed to avoid just this problem.
Known as novel oral polio vaccine type 2 (nOPV2), it was rolled out 2 years ago this month, and public health experts have been closely monitoring whether its use could also spark outbreaks on rare occasions.
“It’s disappointing but not entirely unexpected,” says Aidan O’Leary, who heads GPEI. To Simona Zipursky of the World Health Organization, who co-chairs GPEI’s nOPV2 working group, the question for the past 2 years has been when, not whether, such cases would occur. “But you always hope you are wrong,” she says.