The rapid spread of the omicron variant could be a major setback for efforts to reverse sharp inequality in the accessibility of effective coronavirus vaccines around the world, public health experts warn.That could mean longer delays for vaccination programs in poor countries, leaving large populations unprotected and prolonging the circumstances under which experts say dangerous variants are most likely to emerge.

A subsequent increase in demand for mRNA vaccines and booster doses, which appear to offer the best protection against omicron, would drive a widening of the divide, as rich countries accrue more doses and poor ones continue to go without or face long delays and uncertainty.

After a global run on doses, logistics have supplanted availability as the chief obstacle to vaccinating the world’s neediest, World Health Organization vaccine director Kate O’Brien told The Washington Post earlier this month. But omicron could bring availability problems soaring back, experts fear.

“The inequalities are worsening,” said Tsion Firew, an assistant professor at the Columbia University Medical Center and adviser to Ethiopia’s Ministry of Health. “Especially as the omicron variant is widely spreading in places like the United States,” which continues to record a rate of per capita deaths among the highest in the world, “we are going to see more vaccine hoarding and we are going to see it become more difficult for low-income countries to access” doses, she said.

In a news conference on Wednesday, the WHO once again urged countries to wait on administering widespread boosters, and to instead focus on ensuring first and second doses globally.

There’s still much to learn about omicron. But initial studies show that the highly contagious variant more easily evades vaccines, though vaccinated people remain at a far lower risk of developing severe disease.

Coronavirus vaccines also do not appear to protect against omicron equally: Newer mRNA vaccines produced by Pfizer and Moderna seem to be more effective, according to the first round of research, than technologies used by Johnson & Johnson, AstraZeneca, Russia’s Sputnik, and China’s Sinopharm and Sinovac — which produces the world’s most widely used vaccine.