The COVID-19 pandemic is understandably the focus of medical personnel and institutions, but all other infectious diseases haven’t left. However, the non-pharmaceutical interventions (NPIs) imposed against the former have also greatly reduced the cases of the latter.

New research from Princeton University showcases how measures such as compulsory mask-wearing and social distancing have “greatly” reduced the incidence of all infectious diseases such as influenza and respiratory syncytial virus (RSV). However, the authors argue that we should avoid letting this decrease lull us into a false sense of security, as we may simply be seeing a postponement of future outbreaks.

Temporary protection

“Declines in case numbers of several respiratory pathogens have been observed recently in many global locations,” said first author Rachel Baker, an associate research scholar at the High Meadows Environmental Institute (HMEI) at Princeton University.

“While this reduction in cases could be interpreted as a positive side effect of COVID-19 prevention, the reality is much more complex. 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.”

The NPIs applied against the pandemic could lead to an increase in respiratory syncytial virus infections in the future, the team explains. RSV is a virus endemic to the United States and a leading cause of lower respiratory tract infections in infants. The same is true for influenza, they add, albeit to a lower extent.

The team used an epidemiological model based on historic RSV data, factoring in recent downward trends in RSV cases. They then used this model to assess the possible impact of COVID-19 NPIs on RSV outbreaks in the United States and Mexico in the future. All in all, they found that even relatively short periods of NPI measures such as mask use could promote large RSV outbreaks in the future.

Such outbreaks were typically delayed a bit after the end of the NPI use phase; according to the model, we should expect cases to peak around the winter of 2021-2022.

“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 said.

Seasonal influenza would follow the same pattern in the future, but the authors caution that it is much harder to project its behavior in the future due to its habit of evolving rapidly. Here, the availability of vaccines would make “a big difference” says Baker.

Bryan Grenfell, the Kathryn Briger and Sarah Fenton Professor of Ecology and Evolutionary Biology and Public Affairs at HEMI and co-author of the paper calls the drop in influenza and RSV cases “arguably the broadest global impact of NPIs across a variety of human diseases that we’ve seen”. Other diseases could be impacted by these measures as well over the long term, and better understanding these mechanisms can help us stay safe after the pandemic.

Exactly how NPIs influence outbreaks of infectious diseases depends on how they’re implemented and lifted, but also on biological factors — most notably the public “landscape of immunity and susceptibility”. After the 1918 influenza pandemic, the team explains, measles in London shifted from annual cycles to biennial outbreaks after NPI measures were lifted. The authors recommend the use of tools such as serology to better map this susceptibility in order to prevent such life-threatening changes in the future.

The paper “The impact of COVID-19 non-pharmaceutical interventions on the future dynamics of endemic infections” has been published in the journal Proceedings of the National Academy of Sciences.