Pandemic impacts influenza and other respiratory viruses

July 27, 2021

In the United States, the circulation of respiratory viruses was disrupted during the COVID-19 pandemic, but the magnitude, timing and duration of this effect varied among viruses.

During 2020, influenza viruses and RSV circulated at historically low levels. In 2021, influenza continues to circulate at low levels; whereas, RSV activity has been increasing since April 2021, indicating an unusually timed increase in some regions of the country. HCoV and PIV activity is rising to prepandemic levels after notably low circulation, but this HCoV activity is inconsistent with the timing for a typical season. HPMV activity has remained low since March 2020. Although RAdV and RV/EV activity decreased in spring 2020, circulation has reverted to the week-to-week fluctuations at levels similar to those observed before the pandemic.

The duration of the effect of the COVID-19 pandemic and associated mitigation measures on respiratory virus circulation is unknown. Circulation of other respiratory viruses might continue to change as pandemic mitigation measures are adjusted and as prevalence of and immunity to both SARS-CoV-2, the virus that causes COVID-19, and immunity to these other viruses waxes and wanes. In 2020, influenza continued to circulate in the tropics; therefore, resumption of circulation in the United States is possible as global travel resumes.

Every year, it is difficult to predict which influenza viruses might circulate during the next season. In the United States, influenza A (H3N2) viruses continue to be identified, but the diversity of the subclades co-circulating was reduced relative to recent seasons, and globally, few detections of influenza B viruses of the Yamagata lineage were detected during the pandemic. 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.

Lower levels of population immunity, especially among younger children, could portend more widespread disease and a potentially more severe epidemic when influenza virus circulation resumes. As the fall season approaches with schools and workplaces reopening, in addition to the use of recommended everyday preventive actions, clinicians should encourage influenza vaccination for all persons aged ≥6 months.

RAdV and RV/EV activity continued during 2020 and might be returning to prepandemic circulation patterns. Factors contributing to this distinct circulation are unclear but might include the relative importance of different transmission mechanisms, such as aerosol, droplet, or contact, the role of asymptomatic transmission, and prolonged survival of these nonenveloped viruses on surfaces, all of which might make these viruses less susceptible to nonpharmaceutical interventions, such as mask-wearing and surface cleaning. The delay in circulation of PIVs and HCoVs, which circulate at high levels among children, could be related to some schools suspending in-person classes until late winter.

Influenza viruses and human metapneumovirus circulated at historic lows through May 2021. In April 2021, respiratory syncytial virus activity increased. Common human coronaviruses, parainfluenza viruses, and respiratory adenoviruses have been increasing since January or February 2021. Rhinoviruses and enteroviruses began to increase in June 2020.

Clinicians should be aware of increased circulation, sometimes off season, of some respiratory viruses and consider multipathogen testing. In addition to recommended preventive actions, fall influenza vaccination campaigns are important as schools and workplaces resume in-person activities with relaxed COVID-19 mitigation practices. 

The COVID-19 pandemic and subsequent implementation of nonpharmaceutical interventions (e.g., cessation of global travel, mask use, physical distancing, and staying home) reduced transmission of some viral respiratory pathogens.

In the United States, influenza activity decreased in March 2020, was historically low through the summer of 2020, and remained low during October 2020–May 2021 (<0.4% of respiratory specimens with positive test results for each week of the season). Circulation of other respiratory pathogens, including respiratory syncytial virus (RSV), common human coronaviruses (HCoVs) types OC43, NL63, 229E, and HKU1, and parainfluenza viruses (PIVs) types 1–4 also decreased in early 2020 and did not increase until spring 2021. Human metapneumovirus (HMPV) circulation decreased in March 2020 and remained low through May 2021.

Respiratory adenovirus (RAdV) circulated at lower levels throughout 2020 and as of early May 2021. Rhinovirus and enterovirus (RV/EV) circulation decreased in March 2020, remained low until May 2020, and then increased to near prepandemic seasonal levels. Circulation of respiratory viruses could resume at prepandemic levels after COVID-19 mitigation practices become less stringent. Clinicians should be aware of increases in some respiratory virus activity and remain vigilant for off-season increases. In addition to the use of everyday preventive actions, fall influenza vaccination campaigns are an important component of prevention as COVID-19 mitigation measures are relaxed and schools and workplaces resume in-person activities.

The different epidemiologic patterns of respiratory viruses observed during the COVID-19 pandemic in this U.S. surveillance summary raise questions about transmission and prevention, such as the contribution of birth cohort effects, natural immunity, and interventions. Clinicians should be aware that respiratory viruses might not exhibit typical seasonal circulation patterns and that a resumption of circulation of certain respiratory viruses is occurring, therefore an increased index of suspicion and testing for multiple respiratory pathogens remain important. Improved understanding of the role that nonpharmaceutical interventions play on the transmission dynamics of respiratory viruses can guide future prevention recommendations.

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