Low fertility rates, which have persisted since the end of the Great Recession, and the nation’s continuing demographic shift toward an older population also combined to create the smallest population increase in 100 years, said Kenneth M. Johnson, a sociology professor and demographer at the University of New Hampshire. He said he expected the data to show a natural decrease but was surprised at its scale. Natural decrease occurs when a population records more deaths than births.

“I think one of the most important findings is the fact that almost 2,300 counties had more deaths than births in them. That’s unheard of in American history,” Johnson said.

He said the coronavirus’s impact, along with longer-term trends that limited population growth, had created “a perfect storm,” and that one would have to go back at least to the 1918 flu pandemic to find anything like it.

The data also offered statistical backing to widespread anecdotal evidence suggesting that millions of Americans moved out of the nation’s largest cities, including the District, during the pandemic. Whether for safety from infectious disease or convenience during shutdowns, millions traded cities for suburbs or larger suburbs for smaller ones. Many migrated farther into rural counties or resettled to second homes in vacation areas, such as the Catskill Mountains or the Delmarva Peninsula.