Comment by paviva

An alternative explanation is that of "viral interference"[1,2], i.e., the most transmissible virus boosting up viral immunity in the population, and precluding transmission of the less transmissible/fit virus

[1]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5489283/

[2]: https://www.thelancet.com/journals/lanmic/article/PIIS2666-5...