The loss of residents was due to a combination of deaths outweighing births—further exacerbated by the pandemic—an aging population, and an uptick in people moving to less populous or less expensive areas amid a surge in remote working.
Los Angeles County, which has had the highest number of deaths from COVID-19 in the United States, experienced the largest population loss of any county, losing about 184,465 residents in the first year of the COVID-19 pandemic, according to the data.
Three other California counties also saw sizable resident losses: San Francisco County (58,764), Santa Clara County (50,751), and Alameda County (33,797).
California's losses were followed closely by those of New York, which, at one point in spring 2020, became the epicenter of the COVID-19 pandemic in the United States, leaving hospitals overwhelmed.
New York County lost 117,375 residents, according to the data, while Kings County lost 95,022 and Queens County lost 74,321.
Cook County in Illinois, which includes Chicago, also reported big losses, declining by 102,395 residents, according to the data.
Nearly all of the states' population loss was driven by domestic migration, which is when people move from one area of the United States to another, according to the data.
Los Angeles County experienced the greatest net domestic migration loss with 179,757 residents, followed by New York County with 113,642.
"This loss that both California is experiencing and Los Angeles County is experiencing is kind of the perfect storm from a demographic perspective, and all the components that lead to population change are all trending in a downward direction for both the state and Los Angeles."
Overall, nearly three-quarters of counties in the United States experienced a spike in deaths in 2021, with more than 73 percent (2,297) seeing a "natural decrease" (when more deaths occur than births over a given period of time) in population in 2021, up from 45.5 percent in 2019 and 55.5 percent in 2020 at the height of the COVID-19 pandemic, according to the data.
In 2021, all counties in Delaware, Maine, New Hampshire, and Rhode Island experienced more deaths than births, according to the data.
Generally, though, most counties (2,063, or 65.6 percent) experienced "positive domestic migration" from 2020 to 2021, meaning that there were more people entering than leaving those areas.
Among metro areas, 251 (65 percent) experienced population increases between that time period, while 287 (52.9 percent) of the 543 U.S. micro areas saw population increases in 2021.
"The patterns we’ve observed in domestic migration shifted in 2021," said Christine Hartley, assistant division chief for estimates and projections in the Census Bureau’s Population Division. "Even though over time we’ve seen a higher number of counties with natural decrease and net international migration continuing to decline, in the past year, the contribution of domestic migration counteracted these trends so there were actually more counties growing than losing population."