COLUMBIA, MO — While co-infection with more than one respiratory virus was especially a problem this fall and winter, it has happened before, especially in an environment where SARS-CoV-2 is circulating.
In a study involving Walter Reed Army Institute of Research, a team led by the University of Missouri sought to evaluate the prevalence and clinical impacts of SARS-CoV-2 and influenza A virus co-infections during the 2021-2022 influenza season. To do that, SARS-CoV-2-positive samples from 462 individuals in a single geographic area were collected from October 2021 to January 2022. Of those, 152 tested positive for influenza, and the monthly co-infection rate ranged from 7.1% to 48%.
“Compared to the Delta variant, individuals infected with omicron were less likely to be co-infected and hospitalized, and individuals who received influenza vaccines were less likely to become co-infected,” the authors wrote in Virology. “Three individuals had two samples collected on different dates, and all three developed a co-infection after their initial SARS-CoV-2 infection. This study demonstrates high prevalence of co-infections in central Missouri during the 2021-2022 influenza season, differences in co-infection prevalence between the delta and the omicron waves and the importance of influenza vaccinations against co-infections.”1
They pointed out that vaccination appeared to diminish the likelihood of co-infection.
Background information in the report noted that, based on prior studies, hospitalized patients with COVID-19 infections, co-infection with other respiratory viruses may be associated with more severe clinical outcomes and increased mortality among hospitalized patients.
The authors added that, during the three influenza seasons overlapping the COVID-19 pandemic, co-infections of COVID-19 and influenza have been reported in multiple studies, with prevalence ranging from 0.2 to 45.7%. “However, the implications of co-infections in the general population, particularly during the most recent influenza season, remain unclear,” they advised. “To evaluate the prevalence and clinical impacts of SARS-CoV-2 and influenza A virus (IAV) co-infections, we performed a cross-sectional study in central Missouri during the 2021-2022 influenza season and investigated the associations of the latest COVID-19 variants of concern (VOC) with co-infections and clinical outcomes. Additionally, we assessed the association of COVID-19 and influenza vaccines with co-infection.”
The results indicated that the prevalence of influenza co-infection among patients infected with SARS-CoV-2 was 33% between October 2021 and January 2022. The researchers pointed out that was considerably higher than most recent studies, but that the previous studies were conducted in 2020 and did not account for differences between SARS-CoV-2 variants.
“It is plausible that low co-infection rates in previous reports were due to study periods during low influenza activity, as influenza activity was historically low during the 2020-2021 influenza season largely due to non-pharmaceutical interventions implemented for COVID-19 control or additional factors including differences in COVID-19 variant predominance during different phases of the pandemic,” the researchers suggested. “On the other hand, it is also likely that the prevalence of co-infection has been underreported due to the diagnostic focus on COVID-19 during the pandemic. Individuals who are asymptomatic, not hospitalized, and have mild symptoms are typically not tested for influenza virus, and with the co-circulation of SARS-CoV-2 in recent years, many healthcare providers are testing for COVID-19 in individuals experiencing respiratory symptoms without additionally testing for influenza.”
They added that their study suggested that co-infection of influenza with the Omicron variant might have been reduced compared to co-infection with the Delta variant and called for further studies to evaluate whether the Omicron variant is more likely than the Delta variant to act as an antagonist against IAVs, resulting in a lower co-infection risk.
“As new COVID-19 variants continue to emerge, the clinical outcomes of co-infections cannot be predicted and may potentially become more pronounced. Thus, testing for both SARS-CoV-2 and influenza viruses in individuals experiencing influenza-like illness should be encouraged, especially in people with increased risks of complications, as a prompt influenza and/or COVID-19 treatment can potentially help prevent more serious illness and/or hospital admission,” the authors wrote.
The researchers concluded that their results indicate that individuals who received at least one influenza vaccine during the 2020-2022 influenza seasons were less likely to be co-infected with SARS-CoV-2 and IAV.
- Tang CY, Boftsi M, Staudt L, McElroy JA, Li T, Duong S, Ohler A, Ritter D, Hammer R, Hang J, Wan XF. SARS-CoV-2 and influenza co-infection: A cross-sectional study in central Missouri during the 2021-2022 influenza season. Virology. 2022 Nov;576:105-110. doi: 10.1016/j.virol.2022.09.009. Epub 2022 Sep 30. PMID: 36206606; PMCID: PMC9523501.
