Omicron arrived in the United States at a time when 60% of Americans had the protection of COVID-19 vaccines and roughly a third of Americans (including some who’d been vaccinated) had a past infection. Not only was there a high level of population immunity, those who did become ill had access to treatments that weren’t available to people sickened by the initial strain from Wuhan, China, or the Alpha and Delta variants that followed.

Perhaps these are the reasons why those infected with Omicron have tended to experience milder illnesses.

“It’s widely said that Omicron is inherently less pathogenic, but there is no real evidence for that,” said Dr. Christopher Chiu, a COVID-19 researcher at Imperial College London.

“Comparisons with Delta are like apples and oranges,” he said. “Delta was circulating at a time when many were still not vaccinated or previously infected. In contrast, Omicron is largely causing breakthrough infections in people who already have partial protection from immunity conferred by vaccines or infection.”