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The findings, posted this week to the preprint database medrXiv and not yet peer-reviewed, underscore the importance of flu shots this year — both studies suggested that a bad flu season could be avoided if flu vaccination rates increased by 20% to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. They note that the U.S. is already seeing a rise in cases of other respiratory infections, such as respiratory syncytial virus (RSV), "which does not bode well for the coming flu season," Roberts said.
In one of the new studies, led by Kyueun Lee, a postdoctoral researcher at Pitt Public Health, the researchers used a mathematical model called the Susceptible-Exposed-Infected-Recovered (SEIR) model. They simulated influenza epidemics, and levels of population immunity to flu, over multiple seasons, using data from 2009 through 2020.
They predicted that, with the low flu activity seen in the 2020-2021 season, flu hospitalizations would surge to 610,000 in 2021-2022, which is 102,000 more hospitalizations that would be expected to occur if the 2020-2021 season had seen normal levels of flu activity. In a worst-case scenario with a highly transmissible flu strain and low levels of flu vaccination, the model predicted there would be 409,000 more hospitalizations than expected (or more than 900,000 hospitalizations overall.)
But the study found that the predicted increase in flu hospitalizations could be avoided if the percentage of Americans vaccinated against flu increased from the typical 50% to 75%.
The second study, led by Mary Krauland, who studies mathematical modeling at Pitt Public Health, used a different model, called the Framework for Reconstructing Epidemiologic Dynamics, to simulate flu cases and hospitalizations over two sequential flu seasons, and found similar results. The findings suggested that the 2021-2022 flu season could see a 20% increase in flu cases compared with a typical season. Young children (younger than age two) would be particularly at risk for flu in the 2021-2022 season because they are unlikely to have any previous exposure to the disease, the authors said.
But increasing flu vaccination rates by just 10% could reduce hospitalizations by 6% to 46%, depending on the transmissibility of the flu this season, the study found.
"The 'twindemic' — a coinciding flu and COVID-19 epidemic — overwhelming our hospitals was thankfully avoided last year. But that does not mean it is no longer possible," Roberts said. "If anything, our models show that we should be more concerned this year about the possibility of a surge in COVID-19 hitting at the same time as a massive flu outbreak in areas of the country with low vaccination rates against both diseases."
Originally published on Live Science.
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Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.