By some estimates, the case fatality rate — the share of recorded Covid cases that prove deadly — is one-third lower than it was last summer and one-quarter lower than it was in December. Recorded cases always understate actual infection levels, and the prevalence of at-home testing these days has made that especially true.

To account for those problems, Dr. Dowdy looked at the proportion of reported test results that are positive, a figure known as test positivity. That measure, too, is imperfect, but it reflects the enormous numbers of Americans who recently contracted the virus; some scientists estimate that the current wave of cases is the second-largest of the pandemic.

By his rough calculations, Dr. Dowdy estimated that the ratio of deaths to test positivity fell threefold from the early days of the pandemic to January 2022, and fourfold from January 2022 to this spring.

“What we’re seeing is that the average case of Covid-19 is becoming much milder,” he said.