Even if you were rarely allowed to go to a real island at the time, summer 2020 felt like an island. An island of calm, tossed by the waves of the corona pandemic. Far too few suspected at the time that after the first wave of infections, illness and death had been overcome, a second wave that was several times as high would soon pile up. And yet you remember how the virus suddenly became inconspicuous in the summer and life was halfway normal.
Many are now hoping for the summer of 2021. Perhaps the rising temperatures these days could already have the first positive effects on the development of the number of infections, as in April 2020 when the first wave broke.
Viruses experience seasonal effects
From the point of view of most virologists, however, you shouldn’t expect too much from spring and summer. The warm seasons are certainly causing problems for the corona virus. The transmission dynamics tend to weaken in summer, but not so much that it would be of real help.
Such a seasonal effect is known from many viruses. The flu, numerous respiratory diseases and gastrointestinal infections occur more frequently in winter, but in summer you usually only catch a cold if you have bathed too long in cold water or if you let the warm summer rain in your clothes get cold.
There are several reasons for this. “The heat changes the fat molecules in the virus’ s shell in such a way that they can burst,” says virologist Stephanie Pfänder from the University of Bochum. In addition, UV radiation, which is stronger in summer, attacks the genetic makeup of the virus and can inactivate it. And the high humidity makes the viruses’ travel vehicles, the droplets and aerosols, so heavy that they sink to the ground and people can no longer fly up their noses and throats so easily; It also moisturizes the mucous membrane of the nose and throat and promotes their protective function. In addition, there is the behavior of people who gather less in poorly ventilated rooms in summer. And finally, in summer, the immune system is fitter and more geared towards fighting new pathogens.
According to Christian Drosten, the summer of 2020 benefited from the low number of cases in the spring: “That is no longer the case.”
But despite these many positive factors, the spread of Sars-CoV-2 is less influenced by the seasons than that of other viruses. The virologist Christian Drosten from the Berlin Charité therefore recently dampened hope for the warm seasons in the Spiegel: “The fact that we had such a relaxed summer in 2020 probably had to do with the fact that our case numbers remained below a critical threshold in the spring,” he said, “but that is no longer the case.” In Spain, for example, the number of cases rose again in the summer after a lockdown – despite the heat. Other regions of the world are also severely affected by Corona despite high temperatures all year round, for example tropical Manaus in Brazil.
And finally, you shouldn’t forget the new virus variants. Through its various effects, summer may curb the spread of the virus and lower the R value by around 0.5. But the new mutants of Sars-CoV-2 are more easily transmitted from person to person; their R-value is estimated to be 0.3 to 0.7 higher. The summer effect would be gone by that alone.