Is yellow fever the next pandemic? The World Health Organization confirmed on Monday that recent cases in the Democratic Republic of the Congo are connected to an outbreak that began several months ago in Angola, the largest in that country in 30 years. Now some infectious disease researchers are worried that that low vaccination rates in Africa could make it hard to contain the disease.

The outbreak has killed 225 in Angola since December, and 21 so far in the DRC. There is a vaccine for yellow fever, which is endemic in certain parts of South America and Africa. But the Aedes aegypti mosquito—the same insect that transmits Zika virus and dengue fever—thrives in cities, and especially in urban slums, where many people are unvaccinated. That, combined with the rapid growth of urban areas in the tropical and subtropical regions where the mosquito thrives, has scientists worried that if the disease is allowed to spread to other parts of Africa and perhaps Asia, it could overwhelm the world’s supply of vaccines.

(Read more: Nature, “Inside the Mosquito Factory That Could Stop Dengue and Zika,” “The Extinction Invention”)