Pandemic shifted the timing of respiratory infections and mortality

Researchers at the Max Planck Institute for Evolutionary Biology give an explanation why influenza, RSV and other respiratory infections occurred unusually early following the disruption caused by the COVID-19 pandemic. Seasonal mortality shifted almost in parallel. The findings have implications for the timing of vaccination campaigns and provide evidence of a link between respiratory infections and cardiovascular disease.

To the Point:

• Season shifted: Respiratory infections peaked eight to twelve weeks earlier following the pandemic disruption.
• Mortality followed: Seasonal cardiovascular mortality also shifted markedly earlier.
• Vaccination timing matters: After a disrupted season, vaccination campaigns may need to start earlier.

Respiratory infections in Germany usually follow a remarkably stable seasonal rhythm. Before the COVID-19 pandemic, influenza, RSV and other respiratory diseases typically peaked in February or March. During and after the pandemic, this pattern changed considerably. Michael Sieber and Arne Traulsen from the Max Planck Institute for Evolutionary Biology investigated why.

They analysed publicly available data on respiratory infections, hospitalisations and deaths in Germany and combined these data with an epidemiological model. In the 2021/22 and 2022/23 seasons, the onset and peak of the respiratory disease season occurred eight to twelve weeks earlier than before the pandemic. RSV activity increased as early as September, severe respiratory infections peaked in November 2021, and the 2022/23 influenza wave reached its peak in December.

According to the model, the explanation is not simply weather or season. A more important factor is how many people are susceptible to an infection at the beginning of a season. During the contact restrictions of 2020/21, the transmission of many respiratory pathogens was strongly reduced, with influenza and RSV almost completely absent. When these pathogens returned, they encountered a larger susceptible population and were therefore able to spread earlier within their seasonal window.

Importantly, this does not mean that individuals had a ‘weakened immune system’. The study provides no evidence for an individual ‘immunity debt’. Instead, it describes an effect at population level: when an epidemic season is skipped, the number of people susceptible to a particular pathogen changes, affecting the timing of the following epidemic.

Influenza and RSV have since moved back towards their previous seasonal rhythm. In the 2023/24 and 2024/25 seasons, their peaks again occurred mainly between late January and March. This suggests that the post-pandemic shift was temporary.

A second finding is particularly striking. The seasonal peak in mortality shifted alongside respiratory infections. Before the pandemic, mortality in Germany generally peaked in February or March. During the pandemic years, this peak moved forward to December or early January. A similar pattern was found for deaths from cardiovascular disease, with their seasonal peak shifting by two to three months and later moving back towards its previous timing.

This close temporal relationship supports the hypothesis that respiratory infections can contribute to the risk of severe cardiovascular events. It does not, however, demonstrate a direct cause-and-effect relationship. The study analyses patterns at population level and cannot determine whether an infection immediately preceded a heart attack or stroke in an individual patient. Studies linking individual infection histories with cardiovascular events would be needed to establish this relationship more precisely.

For prevention, the timing of epidemic waves has practical consequences. After an unusually mild or almost absent season, the following epidemic may begin earlier. Vaccination campaigns should therefore consider the current epidemiological situation rather than relying exclusively on a fixed seasonal calendar.

The authors highlight influenza vaccination in particular. COVID-19 showed a different seasonal pattern: in 2023/24 and 2024/25, seasonal COVID-19 activity began as early as late August. The study therefore suggests that a vaccination campaign beginning only in late autumn may be comparatively late for COVID-19.