COVID-19 safety measures may only delay future outbreaks of flu, respiratory illnesses

PRINCETON, N.J. — Until a vaccine proves effective, social distancing and face masks are playing the key roles in preventing COVID-19. So what happens when the virus is gone? Although scientists find fewer people are getting sick from other illnesses right now, a team from Princeton University says these safety measures may only be delaying outbreaks the population normally sees during flu season.

Their study reveals that non-pharmaceutical interventions (NPIs) like mask wearing and avoiding large gatherings have resulted in fewer cases of influenza and respiratory syncytial virus (RSV) during the pandemic. The worry is that less exposure to these viruses now may make their eventual return even harder to deal with.

“Declines in case numbers of several respiratory pathogens have been observed recently in many global locations,” says first author Rachel Baker in a university release.

“While this reduction in cases could be interpreted as a positive side effect of COVID-19 prevention, the reality is much more complex,” the associate research scholar at the High Meadows Environmental Institute (HMEI) adds. “Our results suggest that susceptibility to these other diseases, such as RSV and flu, could increase while NPIs are in place, resulting in large outbreaks when they begin circulating again.”

Predicting future outbreaks

Study authors believe NPIs may actually cause a future rise in RSV cases. Respiratory syncytial virus is the leading cause of lower respiratory-tract infections among infants in the United States.

“Although the detailed trajectory of both RSV and influenza in the coming years will be complex, there are clear and overarching trends that emerge when one focuses on some essential effects of NPIs and seasonality on disease dynamics,” Princeton professor of geosciences and study co-author Gabriel Vecchi says.

The study examined historical data on RSV cases to project how social distancing policies during COVID may affect upcoming outbreaks in North America. Their results reveal that even short periods of time observing these safety guidelines may cause a large RSV outbreak in the future. A projected outbreak would likely be delayed, researchers say, until after the public ends its NPI practices. The peak of this new outbreak would likely hit in winter of 2021-22.

“It is very important to prepare for this possible future outbreak risk and to pay attention to the full gamut of infections impacted by COVID-19 NPIs,” Baker warns.

What happens to the flu season?

Study authors also investigated the impacts of social distancing on the seasonal flu outbreak. Their findings reveal a similar outbreak may occur, but researchers add that predicting flu is harder to do. This is due to the uncertainty surrounding which strains of influenza will be circulating each year. A lot also depends on how well the annual flu vaccines fight off future strains of the virus.

“For influenza, vaccines could make a big difference,” Baker explains. “In addition, the impact of NPIs on influenza evolution is unclear but potentially very important.”

“The decrease in cases of influenza and RSV — as well as the possible future increase we project — is arguably the broadest global impact of NPIs across a variety of human diseases that we’ve seen,” co-author Bryan Grenfell adds. “NPIs could have unintended longer-term impacts on the dynamics of other diseases that are similar to the impact on susceptibility we projected for RSV.”

Learning from history

The study points to similar pandemics which saw non-pharmaceutical interventions play key roles in stopping outbreaks. One of these events was the 1918 influenza pandemic. Measles data from London also reveals how the illness changed from an annual event to a biennial outbreak after officials started imposing strict public safety measures.

“The future repercussions of NPIs revealed by this paper hinge on how these measures change the landscape of immunity and susceptibility,” co-author C. Jessica Metcalf concludes.

The study appears in the Proceedings of the National Academy of Sciences.

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