The identification of a new, highly-mutated variant of the coronavirus virus in southern Africa this week highlights the risk to global public health posed by large, unvaccinated populations in the developing world, where countries have struggled to get immunizations and the virus is spreading and evolving.

Overall, just 7% of people in Africa are fully vaccinated, compared with 42% of the global population, according to Our World In Data, a project based at Oxford University. In Europe and the U.S., vaccination levels are 67% and 58%.

That disparity, public-health officials say, helped open the door to the rise of this latest variant, which has more than 50 mutations—including 30 on the virus’s spike, a main target for vaccines—from the one that emerged in Wuhan, China, two years ago.

World Health Organization scientists on Friday declared the new strain, which they named Omicron, a “variant of concern” and said evidence suggests that it is more contagious and that it is able to infect people who have already had Covid-19 or been vaccinated.

About a third of India’s population has now been fully vaccinated, yet vast areas of the developing world remain even further behind. Africa’s most populous country, Nigeria, has fully vaccinated just 1.7% of its 206 million people. Africa’s second-most populous country, Ethiopia, currently in the grips of a civil war, has covered just 1.2%.

Some public-health officials say they are worried the world risks being pulled into a dangerous cycle in which worrying new variants emerge in unvaccinated populations, prompting more highly-vaccinated countries to order up booster shots, making it more difficult to get doses to the developing world.

“We’re looking into a situation where high-income countries will keep getting regular boosters, while people in low-income countries haven’t even had their first dose,” said Alexandra Phelan, an adjunct professor of global and public-health law and ethics at Georgetown University.

To help close the gap, the Biden administration in May said it would support temporarily suspending patents and other intellectual property behind the vaccines. The idea was to let developing countries produce their own shots, for their own people.

Proponents of that, including South Africa and India, have been unable to reach agreement with opponents, including the EU, which says lifting patent protections would discourage investment in pharmaceutical research while not doing much to ease the pandemic.