Monday, July 26, 2021

MMWR: Changes in Influenza and Other Respiratory Virus Activity During the COVID-19 Pandemic

Credit CDC FluView


The remarkable lack of Influenza, and other non-COVID respiratory diseases, globally since March of 2020 is well depicted in the above graphic.  The colored lines show levels of ILI (Influenza-like Illness) in the U.S. during previous winters, and the flatline of red triangles shows the ILI rate for the 2020-2021 `flu season'. 

Between social distancing, face mask wearing, enhanced hand hygiene, and the suspected natural suppression of non-COVID viruses due to `viral interference' (see When Epidemic Viruses Collide), the world's population has been largely spared from flu, RSV, non-polio enteroviruses (D68, EV71, A6, etc.), and even outbreaks of measles and scarlet fever for well over a year. 

While a welcome respite, particularly as hospitals have been slammed with COVID cases, there is a downside; without occasional exposure to these viruses, our individual levels of acquired immunity are expected to wane.  
Now, as countries decide to loosen their social contact restrictions - as is occurring across much of the United States and in the UK - the risks for seeing new epidemics of RSV, measles, influenza, and other (primarily) respiratory viruses goes up (see June 2020's COVID-19, The Next Flu Season, And The Temporary Immunity Hypothesis)

Not only can this make the diagnosing, and isolation, of COVID cases more difficult, there are studies suggesting that coinfection of influenza and COVID may produce more severe illness and less favorable outcomes (see PHE Study: Co-Infection With COVID-19 & Seasonal Influenza).

Over the past 60 days we've seen a marked increase in RSV - both in the United States (see CDC HAN) and in places like New Zealand, Denmark, and the UK - despite this being the `off-season' for RSV for most of the countries. 
UK PHE Warns On A Summer Uptick in RSV Infections
SSI: Denmark Also Reporting A Summer Surge In RSV

ESCMID: When Respiratory VIruses Return & New Zealand's RSV Resurgence

This recent uptick in non-COVID respiratory infections has led to several warnings over what might transpire this fall and winter, including mid-July's UK Academy Of Medical Sciences: Looking Ahead To COVID-19 Over Winter 2021/22 & Beyond and a commentary appearing today in the CMAJ called Potential resurgence of respiratory syncytial virus in Canada.

While a return to wearing face masks or social distancing due to another wave of COVID could delay the timing - the eventual return of influenza and other respiratory viruses is all but assured.  The only unknowns are when, and how bad they will be. 

All of which brings us to a new release from the CDC's MMWR which looks at the changes we've seen in influenza and other respiratory virus activity, and discusses what we might expect in the months ahead. 

Due to its length I've only included the link, summary, and discussion section (bolding mine). Follow the link to read it in its entirety.  I'll have a brief postscript when you return. 

Changes in Influenza and Other Respiratory Virus Activity During the COVID-19 Pandemic — United States, 2020–2021 
 Weekly / July 23, 2021 / 70(29);1013–1019

Sonja J. Olsen, PhD1; Amber K. Winn, MPH2; Alicia P. Budd, MPH1; Mila M. Prill, MSPH2; John Steel, PhD1; Claire M. Midgley, PhD2; Krista Kniss, MPH1; Erin Burns1; Thomas Rowe, MS1; Angela Foust1; Gabriela Jasso1; Angiezel Merced-Morales, MPH1; C. Todd Davis, PhD1; Yunho Jang, PhD1; Joyce Jones, MS1; Peter Daly, MPH1; Larisa Gubareva, PhD1; John Barnes, PhD1; Rebecca Kondor, PhD1; Wendy Sessions, MPH1; Catherine Smith, MS1; David E. Wentworth, PhD1; Shikha Garg, MD1; Fiona P. Havers, MD2; Alicia M. Fry, MD1; Aron J. Hall, DVM2; Lynnette Brammer, MPH1; Benjamin J. Silk, PhD2 

What is already known about this topic?

Nonpharmaceutical interventions introduced to mitigate the impact of COVID-19 reduced transmission of common respiratory viruses in the United States.

What is added by this report?

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.

What are the implications for public health practice?

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.


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. Among each group of viruses, changes in the circulation of specific species and types warrant further assessment.
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 (3). Every year, it is difficult to predict which influenza viruses might circulate during the next season (4). 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 (5).

RAdV and RV/EV activity continued during 2020 and might be returning to prepandemic circulation patterns (6,7). 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 (8,9). 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. However, the relative absence of HMPV, which affects a similar age group as RSV (i.e., children aged <2 years) is unexplained. The unusual timing of rising RSV detections was also observed in Western Australia (10).

The findings in this report are subject to at least three limitations. First, changes in health-seeking behaviors during the pandemic (e.g., designated testing sites for COVID-19) might have contributed to a decrease in reported respiratory virus activity if routine health care visits were not made to health care providers who participate in surveillance. Testing for respiratory viruses was somewhat reduced during 2020–2021 but was higher than typically seen during periods of low virus activity. In addition, the detection of sporadic novel influenza viruses and increases in levels of circulation of other respiratory viruses attest to systems’ effectiveness. Second, each test result was independently reported, therefore the role of virus-virus interactions on activity could not be examined. Finally, some viral groupings (e.g., RV/EV) are large and might obscure type-specific patterns.

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.

Charisse Cummings, Rachel Holstein, Stacy Huang, Alissa O’Halloran, Rishika Parikh, Kyung Park, Carrie Reed, Sandra Seby, Dawud Ujamaa, Influenza Division, National Center for Immunization and Respiratory Diseases, CDC; Rebecca Dahl, Meredith McMorrow, Michael Whitaker, Division of Viral Diseases, National Center for Immunization and Respiratory Diseases, CDC.

          (Continue . . . )

The feared `twindemic' of COVID and influenza wasn't realized last year, and perhaps it won't happen  this year either.  But we are in uncharted territory, and nobody knows exactly how COVID - and other respiratory viruses - will play out in the months ahead. 

We do know that even without a pandemic strain, influenza can sometimes deliver a hellish flu season, as it did in over the winter of 2017-18 (see 2018's CDC: The Estimated Burden Of Last Year's Flu Season).

The good news is, there are relatively simple things we can do to reduce the impact of influenza, and other seasonal respiratory viruses, when they eventually do return.  We can get the yearly flu shot, we can continue to wear face masks in public and avoid crowds during outbreaks, we can be scrupulous in our hand hygiene, and we can choose not to go to work, school, or out in public if we are `sick'. 

Things which the past 18 months have shown help to greatly reduce the spread of COVID and seasonal viruses - and frankly things we should be doing each winter out of consideration for others - if not for our own protection.

Lest you think the public wearing of face masks beyond the pandemic is impractical, this has been common practice in Asian countries for years, long before COVID emerged (see 2019's HK CDW: Surgical Masks For Respiratory Protection).

Figure 2 - How to wear a surgical mask. (Source: The Centre for Health Protection of the Department of Health.

We now know how to reduce the burden of seasonal flu, and other respiratory viruses, even after the COVID pandemic ends.  All that remains is finding the will to do so.