Although the pandemic has mostly killed older Americans, the greatest percentage increase in excess deaths has occurred among adults ages 25 to 44, the analysis found.
While the number of deaths among adults ages 45 to 64 increased by 15 percent, and by 24 percent among those ages 65 to 74, deaths increased 26.5 percent among those in their mid-20s to mid-40s, a group that includes millennials.
Among those in the youngest age group, under 25, deaths were 2 percent below average.
People of color also had large percentage increases in excess deaths, compared with previous years. Hispanics experienced a 54 percent increase, while Black people saw a 33 percent rise. Deaths were 29 percent above average for American Indians or Alaska Native people, and 37 percent above average for those of Asian descent.
By comparison, the figure for white Americans was 12 percent, according to the analysis.
The report reviewed deaths from Jan. 26 to Oct. 3 of 2020, and used modeling to compare the weekly tallies with those of corresponding weeks in 2015 through 2019.
The researchers estimated that 299,028 more people than expected died in the United States during that period, with 198,081 deaths attributable to Covid-19 and the rest to other causes.
That estimate is significantly higher than the 216,025 coronavirus deaths officially reported by the C.D.C. as of Oct. 15. (The figure now is nearing 221,000, according to a database maintained by The New York Times.)
Other researchers have also found greater deaths over all during the pandemic. A study published July 1 in JAMA Internal Medicine reported that deaths from all causes in the United States increased by 122,000 from March 1 to May 30, a figure that was 28 percent higher than the deaths attributed solely to Covid-19 during that period.