Scientists don’t fully understand Covid’s cycles, but the explanation probably involves some combination of the virus’s biological qualities and the size of a typical human social network. After about two months, an outbreak of earlier variants began to burn out, much as a forest fire would.

Omicron is so contagious that it spreads more quickly. This rapid spread may also mean that it more quickly reaches most of the people who happen to be vulnerable to being infected by it. Omicron’s brief boom-and-bust cycle is now “a familiar pattern,” Joseph Allen of Harvard’s School of Public Health says.

Ali Mokdad, a professor of health metrics sciences at the University of Washington in Seattle, told the The Associated Press that he believed the true number of U.S. cases — including those not included in any official tally — has already peaked, probably last week. “It’s going to come down as fast as it went up,” he predicted.