Almost three-fourths of all U.S. counties reported more deaths than births last year, a development largely caused by the pandemic, which contributed to a dramatic slowing in the overall population growth of the nation, according to data released Thursday by the Census Bureau.

Low fertility rates, which have persisted since the end of the Great Recession, and the continuing demographic shift toward an older population also combined to create the smallest population increase in 100 years, said Kenneth Johnson, a sociology professor and demographer at the University of New Hampshire.

Johnson 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,” he said.

He said the impact of the coronavirus, along with other 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 largest cities in the nation, including the District, during the pandemic.