> Why is Sweden seeing a second wave as well? Why did the number of infections soar, while it should theoretically have remained constant?

I think we can now conclude that covid19 behaves seasonally, just like other respiratory viruses.

But nobody knows exactly why. There are many theories, but none universally accepted: Summertime higher vitamin D levels from sunlight, maybe this gives better immune defense. Sunlight disinfects surfaces. Colder air is dry, respiratory droplets stay afloat longer and spread better in dry air. People's mucous membranes stay healthier in more humid air, maybe they are more susceptible for virus attacks in dry air. Maybe the patterns of how much people meet each other indoors is different in cold weather.