One study predicted that there could be 100,000 to 400,000 more flu hospitalizations in the 2021-2022 flu season compared with a typical season.

The findings, posted this week to the preprint database medrXiv and not yet peer-reviewed, underscore the importance of flu shots this year — both studies suggested that a bad flu season could be avoided if flu vaccination rates increased by 20% to 50% compared with a typical year. “Vaccinating as many people against flu as possible will be key to avoiding this scenario,” Dr. Mark Roberts, director of the Public Health Dynamics Laboratory at the University of Pittsburgh Graduate School of Public Health, and senior author of both studies, said in a statement.

Last year had historically low levels of flu activity, likely due to measures for preventing COVID-19, such as social distancing, school closures, mask wearing and reduced travel. During the 2020-2021 flu season, the overall flu hospitalization rate in the U.S. was only about 4 hospitalizations per 100,000 people; compared with the usual rate of 70 hospitalizations per 100,000 people. Flu-related deaths dropped by 95%, the researchers said.

This means that the U.S. population “missed the opportunity to establish or boost their immunity [to the flu] for the future influenza season,” which raises the concern that the flu could make a comeback when preventive measures for COVID-19 are lifted, the researchers said.