Study Results Show High Prevalence of COVID-19, Flu Co-Infections in 2021-2022 Season


Analysis of central Missouri discovers differences in co-infection occurrence between Delta and Omicron variants.

Central Missouri saw a high prevalence of COVID-19 co-infections during the 2021-2022 flu season, with a monthly co-infection rate as high as 48% among individuals with COVID-19, according to investigators from the University of Missouri School of Medicine.1

In the study, 462 patients at University of Missouri Health Care who tested positive for COVID-19 were also tested for influenza. Of those who were positive for COVID-19, 33% were also positive for influenza.1

“Co-infection in our samples peaked in October 2021 at 48% when the Delta variant was dominant and reached the lowest point at 7.1% in January 2022 when the Omicron variant prevailed,” Henry Wan, PhD, professor of molecular microbiology and immunology, veterinary pathobiology, electrical engineering, and computer science, said in a statement.1

Of the 462 individuals with COVID-19, 51% had the Delta variant, and 38% had the Omicron variant.1

Nasal swabs were collected between October 1, 2022, and January 27, 2022, which included individuals who were in the emergency department, urgent care, and inpatient and outpatient.2

Three individuals received 2 swabs that were collected on different dates, and 1 individual had 2 swabs on the same day.

Investigators found that those who were infected with the Omicron variant and those who received at least 1 influenza vaccine during the 2020 through 2022 influenzas seasons were less likely to become co-infected with both COVID-19 and influenza.1

Additionally, these individuals were less likely to be hospitalized.1

“Despite low flu vaccine effectiveness for the 2021-2022 season, which was estimated at less than 16%, our study highlights the importance of influenza vaccinations, as they appear to not only offer some protection against influenza infections but importantly, against COVID-19 and flu co-infections,” Wan said.1

It was reported that co-infection was not associated with clinical outcomes, including cardiac symptoms, gastrointestinal symptoms, hospitalizations, or respiratory symptoms.2

Future studies conducted among a more diverse population and in a wider geographic area should be used to determine the prevalence of overall COVID-19 and influenza co-infections, as well as the effectiveness of both COVID-19 and influenza vaccines, investigators said.1

“Testing for both flu and COVID-19 viruses in patients experiencing symptoms of respiratory illness and vaccinations against both viruses should continue to be encouraged,” Wan said.1

Study limitations included the single geographic population. Additionally, all individuals tested positive for COVID-19, but there were no individuals with just an influenza infection, so the effectiveness of both COVID-19 and influenza vaccinations against co-infections were unclear.2

Finally, because those who were included in the study were from a health care facility, they were more symptomatic, making them more likely to seek medical care compared with those who were asymptomatic.2


1. Study finds high prevalence of COVID-19 and flu co-infections during 2021-2022 flu season. News release. EurekAlert. November 2, 2022. Accessed November 10, 2022.

2. Tang CY, Boftsi M, Staudt L, et al. SARS-CoV-2 and influenza co-infection: a cross-sectional study in central Missouri during the 2021-2022 influenza season. Virology. 2022;576:105-110. doi:10.1016/j.virol.2022.09.009

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