In 2021 and 2022, the proportion of hospitalized COVID-19 and influenza patients admitted to a US intensive care unit (ICU) were similar, but COVID-19 patients admitted during the Omicron BA.5 variant period were more likely to die in the hospital, according to a study published late last week in Open Forum Infectious Diseases.
A team led by US Centers for Disease Control and Prevention (CDC) researchers analyzed medical records and COVID-19 and flu surveillance network data from selected counties in 14 states from October 2021 to September 2022.
"After very low levels of influenza circulation during the first year of the COVID-19 pandemic, circulation increased during the 2021-2022 season," the researchers wrote. "The epidemiology of COVID-19 has also changed during the pandemic with circulation of new variants and increasing population-level immunity."
Only 18- to 49-year-olds had higher death rate amid Omicron
In total, 5,777 and 2,363 eligible adults were hospitalized for COVID-19 and flu, respectively. Amid the Delta variant-predominant period (October to December 2021), 1,632 were hospitalized, compared with 1,451 during the Omicron BA.5 period (June to September 2022). During the 2011-22 respiratory virus season, 2,363 people were hospitalized for flu.
During the Delta- and Omicron BA.5- predominant periods, 29.6% and 70.3% of hospitalized adults, respectively, completed at least a primary COVID-19 vaccination series, while 56.2% of flu patients were current on seasonal flu shots.
Continued monitoring of severity trends is warranted as new variants circulate and population-level immunity changes.
The proportion of COVID-19 patients who were admitted to an ICU, received invasive mechanical ventilation/extracorporeal membrane oxygenation (IMV/ECMO), or died in the hospital declined from the Delta to the Omicron BA.5 period. A similar percentage of Omicron BA.5 and flu patients were admitted to an ICU (15.5% vs 13.3%) and received oxygen via high-flow nasal cannula (7.1% vs 9.5%), noninvasive positive pressure ventilation (11.7% vs 14.4%), or IMV/ECMO (5.9% vs 4.2%).
Relative to flu patients, those infected with Omicron BA.5 had higher rates of vasopressor use (8.2% vs 4.8%), renal replacement therapy (5.2% vs 3.4%), and in-hospital death (4.6% vs 2.6%). In-hospital death rates were higher only for those aged 18 to 49 years during the Omicron BA.5 period; there was no difference in death rates for any other age-group between Omicron BA.5 and flu.
Role of vaccination, previous infection
The reduced disease severity amid Omicron BA.5 predominance is likely multifactorial, including increased population-level SARS-CoV-2 immunity from vaccination and previous infection, broader availability of effective drugs, and refined clinical management strategies, the study authors said.
"Despite declines in COVID-19 severity, this analysis demonstrates that both viruses continue to cause severe disease among hospitalized patients, and optimizing disease prevention and treatment strategies like vaccinations, antiviral medications, and use of non-pharmaceutical interventions may attenuate severe disease," the researchers wrote. "Continued monitoring of severity trends is warranted as new variants circulate and population-level immunity changes."
