Physical distancing, wearing face masks, proper handwashing, and other safety measures have not only prevented further spread of the coronavirus, but they have also avoided other respiratory diseases. However, researchers predict that these measures are only increasing people's susceptibility to respiratory diseases, resulting in large outbreaks after the pandemic.
A team from Princeton University assessed the impact that nonpharmaceutical interventions (NPIs) have on circulating infections in a published study in the Proceedings of the National Academy of Sciences. They predict that in the future, there may be large outbreaks of influenza and respiratory syncytial virus (RSV).
Respiratory Illnesses
In 1918, the influenza pandemic affected nearly one-third of the global population. Today, the flu season occurs yearly, typically between late fall and spring.
The authors also noted that evidence from the 1918 influenza outbreak "suggests that NPIs may have reduced measles transmission by 38%." As a result, the annual measles outbreak became biannual after the historic pandemic.
RSV on the other hand, "is the leading respiratory cause of hospitalization in infants and young children in the United States and the world," according to a 2016 study. Moreover, premature babies and newborns with chronic lung disease or congenital heart disease are at most risk of developing RSV.
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Predicting Future Outbreaks
According to surveillance data, the team estimated that this year, RSV transmission had gone down by 20% in the United States. "As susceptibility increases over the NPI period," wrote the authors, there may be peak outbreaks of the two respiratory illnesses next winter.
Rachel Baker said that the decline of respiratory illnesses was also observed around the world. While it is a positive consequence of the ongoing pandemic and spared thousands of people from getting infected, "the reality is much more complex." When respiratory diseases circulate again, people will be more vulnerable to infection.
Despite how complex the factors are in predicting RSV and influenza outbreaks, there is a clear, general trend on how NPIs affect seasonal diseases, as explained by Professor Gabriel Vecchi. To predict future outbreaks of RSV after the pandemic in the U.S. and Mexico, the team used an epidemiological model.
As a result, short periods of NPI measures could be followed by larger RSV outbreaks later on. Baker emphasized the importance of preparing for future outbreaks not only of RSV but other infections that coronavirus NPIs have avoided.
Seasonal influenza outbreaks would most likely have the same outcome, said the team. However, predicting the seasonal flu is more complex due to viral evolution and available vaccines which are only 40% to 60% effective.
"The impact of NPIs on influenza evolution is unclear but potentially very important," said Baker. "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," said Bryan Grenfell. Professor C. Jessica Metcalf concluded that the study reveals how NPIS during pandemics can shift "the landscape of immunity and susceptibility."
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