SARS-CoV-2 Influenced Other Viruses in Different Ways

By Frank P. Esper, MD

The SARS-CoV-2 pandemic and its countermeasures disrupted the circulation of other respiratory viruses, but some more profoundly than others. Understanding these patterns will be important in preparing for the next pandemic.

My team analyzed virus testing data from more than 1.2 million Cleveland Clinic patients in Ohio, from December 2014 through May 2024. Thus, we captured about five years prior to the COVID-19 pandemic and four years into it. We found that nearly all viruses other than SARS-CoV-2 disappeared during the early phase of the pandemic in 2020, when distancing and masking were widely practiced.

Once the vaccine became available and people began re-emerging from lockdown, viruses that typically circulate in the spring, such as metapneumovirus and parainfluenza 3, and those that circulate year-round, including rhinovirus and adenovirus, quickly returned to their usual patterns.

A surprising new shift

But as schools re-opened in the fall of 2021, we observed some very odd behavior with the winter viruses, including influenza A and respiratory syncytial virus (RSV). They shifted their seasonality significantly and took much longer to return to normal.

The most obvious change, which led us to do the study in the first place, was that RSV appeared in August. This really took us by surprise because here in Northern temperate climates like Ohio, RSV typically is a January-February virus. It likes the cold. We had always assumed that it really couldn’t live in warm temperatures.

Yet, there it was, and it came in a huge wave. Our children’s hospital had to readjust accordingly, because the summer is typically slower in respiratory/pneumonia admissions.

Our study data backed up our anecdotal observations, that the winter viruses circulating at the same time as SARS-CoV-2 were displaced more than those that typically circulate in the spring or year-round. Peak RSV seasonality slowly normalized after 2021, reaching its usual seasonal pattern by 2023-2024. Influenza A first transitioned to a semi-annual pattern before finally normalizing in 2024.

Unpacking the practical implications

This phenomenon has major practical implications. We need to know when to expect the flu and RSV in order to time our vaccinations accordingly. If these viruses are going to show up four and a half months early with the next pandemic, we need to understand and be prepared for this scenario in advance.

While we believe that masking and distancing certainly contributed in large part to the disappearance of non-COVID-19 viruses during the lockdown period, that may not be the only contributing factor. There is also a theory about virus-virus interference, that certain viruses are interfering or preventing transmission of other viruses, while others may be more symbiotic and collaborative with others.

We don’t know about that for sure, but we do know that if people are taking precautions and there’s also some amount of nonspecific viral interference, we might see significant reductions in co-circulating strains during peak times. And if that’s the case, we need to be ready in subsequent years for the viruses to come back at unusual times. We need to have vaccines or antibodies on hand.

This phenomenon could also signal changes in the timing and type of virus testing in the clinic. We don’t normally test for RSV in the summer, but we need to keep that possibility in mind, along with other viruses for which we practice season-based testing.

Of course, we hope that our findings will also result in new guidance from the US Centers for Disease Control and Prevention and from professional medical societies, so that insurers will cover off-season testing.

One major limitation of our research is that we only had pre-pandemic data for our Cleveland headquarters and not for Cleveland Clinic’s Florida facilities. Virus circulation differs in Florida’s tropical climate, so I’d like to be able to analyze data from there going forward. My guess is that viral circulation patterns there would be much different from those here in Ohio.

We’ve been seeing pandemics or at least new global outbreaks about once every nine years. We should do what we can to be as ready as possible.

About the author. Dr. Esper is a staff physician in the Center for Pediatric Infectious Diseases at Cleveland Clinic Children's and Associate Professor of Pediatrics at the Cleveland Clinic Lerner College of Medicine.