Since the start of the COVID-19 pandemic, which has resulted in nearly 1 million deaths in the U.S. alone ¹, familiar respiratory viruses have been found to be surfacing according to novel, unexpected patterns ². These new dynamics are affecting labs and hospitals, as physicians continue to update protocols which had previously reflected a predictable cycle of viral infections. In particular, common respiratory viruses have displayed unusual patterns during COVID-19. 

Respiratory syncytial virus (RSV), a common virus that hospitalizes up to 60,000 children each year and may result in lethal lung infections, typically only results in severe infections in the winter months ³. However, the summer of 2021 saw a surprising surge in RSV infections, which have been on the rise again through the spring and early summer of 2022. Infectious-disease specialists have been revisiting their response to RSV as a result – carefully tracking cases in so doing.  

In parallel, common colds have been more virulent and long-lasting; rhinovirus, a cause of the common cold, has been increasingly ending in patient hospitalization. The flu meanwhile, which re-appeared in December of 2021 after a hiatus through most of 2021, interestingly disappeared again in January 2022 after the emergence of the spread of SARS-CoV-2 Omicron variant. However, the flu has only returned mostly without its Yamagata lineage, the variants of which could be simply lying dormant or have gone extinct ⁴. 

Interestingly, such novel trends have been observed in other regions and countries around the world, from Puerto Rico to South Korea ^5,6. Three potential explanations for these trends have been proposed: decreased immune memory due to distancing and isolation, viral interference, and misdiagnoses. 

Fist, these changes may simply reflect our own shifting patterns of behavior. Humans have co-evolved alongside pathogens, and our regular exposure thereto often allows our immune systems to refresh or update their responses without making us sick. Therefore, our self-isolation and lack of exposure to common viruses may have made us individually more vulnerable to their return. At a population level, this lack of immune memory can result in out-of-season viral surges and surprisingly virulent infections.  

Second, these changing dynamics may reflect the interplay between SARS CoV-2 and other viruses in the form of viral interference. SARS-CoV-2 might push aside its rivals through such mechanisms – explaining as such the January 2022 drop in flu cases alongside the rise in COVID-19 cases.  

Third, and finally, some viral infections may be misdiagnosed as COVID-19. Indeed, some rhinovirus, enterovirus and human metapneumovirus infections may look a lot like COVID-19 and respiratory infections. To prevent misdiagnoses, clinicians need to utilize diagnostic tests 7. 

It is predicted that, mirroring other respiratory viruses, COVID-19 will eventually fall into a pattern of seasonal circulation, with a decreasing force of infection. However, additional research remains to be carried out to more specifically dissect the epidemiological dynamics underpinning the spread of SARS-CoV-2 and other respiratory viruses. Meanwhile, clinicians should be aware of increased and unexpected circulation of a spectrum of respiratory viruses and implement multi-pathogen testing. In the meantime, vaccination, alongside maintaining other behavioral barriers to viral spread, remain of the utmost importance.  

References 

1. WHO Coronavirus (COVID-19) Dashboard | WHO Coronavirus (COVID-19) Dashboard With Vaccination Data. Available at: https://covid19.who.int/.
2. Olsen, S. J. et al. Changes in Influenza and Other Respiratory Virus Activity During the COVID-19 Pandemic — United States, 2020–2021. MMWR. Morb. Mortal. Wkly. Rep. 70, 1013–1019 (2021). doi: 10.15585/mmwr.mm7029a1
3. RSV Trends and Surveillance | CDC. Available at: https://www.cdc.gov/rsv/research/us-surveillance.html.
4. Koutsakos, M., Wheatley, A. K., Laurie, K., Kent, S. J. & Rockman, S. Influenza lineage extinction during the COVID-19 pandemic? Nature Reviews Microbiology (2021). doi:10.1038/s41579-021-00642-4
5. Yum, S., Hong, K., Sohn, S., Kim, J. & Chun, B. C. Trends in Viral Respiratory Infections During COVID-19 Pandemic, South Korea. Emerg. Infect. Dis. 27, 1685 (2021). doi: 10.3201/eid2706.210135
6. Quandelacy, T. M. et al. Reduced spread of influenza and other respiratory viral infections during the COVID-19 pandemic in southern Puerto Rico. PLoS One 17, e0266095 (2022).  doi: 10.1371/journal.pone.0266095
7. Agca, H. et al. Changing epidemiology of influenza and other respiratory viruses in the first year of COVID-19 pandemic. J. Infect. Public Health (2021). doi:10.1016/j.jiph.2021.08.004