I was very surprised to hear recently that many virologists say that there’s only “one strain” of CoV-2.
There is evidence that viruses are slowly but continually fine-tuning their “tropism” and transmission to suit their local environment. The reason this is not clear is that scientists are mainly focussing on changes to the surface proteins of the virus. They should also look at changes in the 3-d structures of the virus’s RNA, including the remarkable s2m structure shown below (from Bill Scott’s lab). This is all explained here (for the specialist) and in the links below.
Human viruses are continually moving around the world, including from the Tropics to northern latitudes, and vice versa. For example, the figure below shows that roughly 40% of influenza A and B strains that were present in Europe in 2012 had been in hot countries one year earlier (explanation here). (I believe that) when viruses move to different climates they need to adapt quite quickly if they’re going to compete with local strains.
It would be extraordinary if CoV-2 strains didn’t start to compete with each other in the same way.
The TDVT Hypothesis, as explained by my friend Brian.
For a general discussion of the seasonality of respiratory viruses, written for the layperson, please see Every winter, colds and flu increase.