It has been one full year since polio was detected anywhere in Africa, a significant milestone in global health that has left health experts around the world quietly celebrating.

The goal had seemed tantalizingly close in recent years, but polio always managed to roar back, particularly in Nigeria. Then officials embraced a vigorous new approach to vaccination and surveillance in that country, hiring thousands of community “mobilizers” to track down the unvaccinated, opening operations centers nationwide to monitor progress and seeking out support from clerics and tribal chiefs.

The result has been remarkable.

The last African case of polio was detected in Somalia on Aug. 11, 2014, the final sign of an outbreak with its roots in Nigeria — the one country where the virus had never been eradicated, even temporarily. But the last case in Nigeria was recorded on July 24, 2014.

Africa has never gone so long without a case of polio. But in an indication of how nervous experts still are that the disease may resurge, even the announcement from the Global Polio Eradication Initiative was tentatively headlined “Is Africa Polio-Free?”