In the temperate climates of the northern and southern hemispheres, respiratory infections have always occurred preferentially in the winter months. Even ancient writers wrote about it. The seasonal fluctuations in the spread dynamics of the SARS-CoV-2 coronavirus were also very pronounced during the coronavirus pandemic. However, the reasons for this seasonal variation are still not fully understood.
On closer inspection, however, it is not the case that respiratory infections only spread in the winter months (see overview article „Seasonality of Respiratory Viral Infections“). There are some respiratory pathogens that are present throughout the year, or especially in the summer. This finding is confusing at first because the route of spread is basically the same for all respiratory infections. The fact that some pathogens spread better in winter and others in summer is presumably due to differences in the pathogens. Let us take a look at some of the known pathogens that cause respiratory infections: For example, coronaviruses, influenza viruses and respiratory syncytial virus (RSV) spread preferentially in winter, adenoviruses and rhinoviruses are found all year round, while certain enteroviruses dominate in summer. The groups differ in some respects, as shown in the table below.
| Virus | Preferential spread | Diameter | Lipid membrane | Glycosylation | Genetic molecule | Variabily | Immunity |
| Influenza-Virus | Winter | 80 - 120 nm | yes | yes | RNA | high | short term |
| Corona-Virus | Winter | 80 - 140 nm | yes | yes | RNA | high | short term |
| RSV | Winter | 120 - 300 nm | yes | yes | RNA | medium | short term |
| Adenovirus | All year | 70 - 90 nm | no | low | DNA | medium | long term |
| Rhinovirus | All year | 24 - 30 nm | no | no | RNA | high | low |
| non-rv Enterovirus | Summer | 30 nm | no | no | RNA | high | long term |
Comparing the physicochemical properties of the viruses, it is noticeable that the viruses that are preferentially found in winter (influenza, corona, RSV) are relatively large because they have a lipid envelope and the proteins on the surface of the virus are also equipped with many sugar molecules ("glycosylation"), which makes it easier for the viruses to penetrate human cells. On the other hand, viruses that are present throughout the year (rhinoviruses, adenoviruses) or preferentially in the summer (non-rhinoviral enteroviruses) do not have a lipid envelope, have few sugar molecules, and therefore have a much smaller diameter.
From an immunological perspective, the viruses that are most prevalent during the winter months are distinguished by their ability to evade the development of long-lasting immunity in humans. This phenomenon can be attributed to the robust glycosylation of the virus surface, which impedes the human immune system's capacity to establish a robust long-term immune memory. Consequently, a previous infection or vaccination provides only temporary protection, lasting for a few months. Following this period, a new infection may occur, although the symptoms are typically less severe than those observed during the initial infection.
The fact that humans can only develop short-term immunity to influenza, corona, and RS viruses is apparently the basic prerequisite for the wave-like spread of these viruses. For a stable wave pattern to occur, however, a second factor must be present that ensures the spread is favored in autumn and inhibited in early summer. What is the source of this seasonal factor?
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