“We do not have the information that’s necessary to make the recommendation on whether or not a booster will be needed,” Soumya Swaminathan, the World Health Organization’s chief scientist, said in a Zoom interview Friday. The “science is still evolving.”

Scientists say comparing the prevalence of Covid to the flu, which kills about 650,000 people globally each year, will become an important yardstick come next fall and winter. Covid has killed more than 3.8 million people since the start of 2020, but vaccinated countries should eventually be able to treat its periodic resurgences in the same way as they do the flu — and make policy decisions accordingly.