Influenza and several other respiratory viruses declined in circulation during the pandemic in 2020, but increased during the spring of 2021, researchers found.

Flu activity declined in March of 2020, remaining low until May 2021, with less than 0.4% of respiratory samples testing positive for influenza per week of flu season, reported Sonja J. Olsen, PhD, of the CDC, and colleagues.

Similar patterns were observed for most other respiratory pathogens, including parainfluenza viruses, respiratory syncytial virus (RSV) and common human coronaviruses, the authors wrote in Morbidity and Mortality Weekly Report.

However, all respiratory pathogens followed this pattern. Both human metapneumovirus and respiratory adenovirus circulated at low levels from 2020 to 2021, while rhinovirus and enterovirus remained low until May 2020 before increasing to near “pre-pandemic levels,” the authors noted.

Experts previously warned of a COVID and flu “twindemic” in fall 2020 and emphasized the importance of flu vaccination to combat it.

But from October 2020 to May 2021, circulating influenza experienced the lowest activity reported since 1997. Interventions, such as masks used to mitigate the transmission of COVID-19, likely caused the circulation of common respiratory viruses to decline, the authors said.

Authors examined virologic data from U.S. laboratories available through the U.S. World Health Organization Collaborating Laboratories System and CDC’s National Respiratory and Enteric Virus Surveillance System, as well as hospitalization data from the Influenza Hospitalization Surveillance Network. They reported on influenza activity from Oct. 3, 2020 until May 22, 2021 and circulating viral activity from Jan. 4, 2020 until May 22, 2021 and compared the data to the last four influenza seasons.

Researchers tested 1,095,080 samples, and 0.2% tested positive for influenza. For 502,782 samples tested by public health laboratories, 0.05% tested positive for influenza. Among these positive samples, 60% tested positive for influenza A and 40% for influenza B.

Of influenza A seasonal viruses with subtype available, 53.8% were A(H3N2) and 46.2% were A(H1N1). Of the subtyped influenza B viruses, 32% were B/Yamagata and 68% were B/Victoria lineages.

The cumulative influenza-associated hospitalization rate was 0.8 for every 100,000 patients in contrast to the past 4 influenza seasons, which ranged from 62 to 102.9 per 100,000.

CDC researchers warned that when viral respiratory circulations are able to resume, they could reach pre-pandemic levels once COVID-19 mitigation practices are dropped or lessened. The authors urged clinicians to stay vigilant regarding off-season spikes in viral respiratory circulation activity and encouraged the public to continue taking preventative actions, such as obtaining fall influenza vaccinations.

“Reduced circulation of influenza viruses during the past year might affect the severity of the upcoming influenza season given the prolonged absence of ongoing natural exposure to influenza viruses,” according to the researchers.