A study released Thursday by the U.S. Census Bureau suggests COVID-19 played a role in lowering the population in more than 73% of U.S. counties last year. 

The bureau’s Vintage 2021 estimates of population and components said the population drop — reported in nearly 3,000 U.S. counties — was what they call a “natural decrease,” caused by more deaths than births in a population over a given time period.  

In a news release, the bureau said the study indicates a drop in births, an aging population and increased mortality intensified by the COVID-19 pandemic led to a rise in the natural decrease. 

The study showed that some counties also experienced population declines attributable to both international and domestic migration. The census showed counties with net international migration loss — more people moving out of the country from the state rather than into the country — were most frequently found in California, Oregon and Mississippi. 

The study showed states with the highest percentages of counties with net domestic migration loss — people moving within the United States — were Alaska, Louisiana and Illinois. 

The study showed that more than 65% of U.S. counties experienced positive domestic migration overall from 2020 to 2021. Arizona’s Maricopa County gained the most residents from domestic migration, followed by Riverside County, California, and Collin County, Texas. Los Angeles County, California, experienced the greatest net domestic migration loss, followed by New York County, New York.  

Christine Hartley, assistant chief of estimates and projections at the bureau, said 2021 saw a shift in domestic migration patterns. She said even with the natural decrease in population and international migration overall because of domestic migration, more counties gained population than lost. 

The bureau said the statistics released Thursday include population estimates and components of change for the nation’s 384 metropolitan statistical areas, 543 micropolitan statistical areas and 3,143 counties. 

Some information for this report was provided by The Associated Press.