One possibility seems to be that the virus could be evolving towards lower mortality rates, possibly in conjunction with other mentioned environmental causes (higher ratio of young people being out and about, Vitamin D). I don't know of any direct evidence for this, but it might be what we'd expect from theory (e.g. SA Frank 1994, PRSB). If, for example, a portion of severity of case is underpinned by genetics of the virus (and not just the host), and these people are more mobile (and have been for months), there could be selection for reduced virulence.
One possibility seems to be that the virus could be evolving towards lower mortality rates, possibly in conjunction with other mentioned environmental causes (higher ratio of young people being out and about, Vitamin D). I don't know of any direct evidence for this, but it might be what we'd expect from theory (e.g. SA Frank 1994, PRSB). If, for example, a portion of severity of case is underpinned by genetics of the virus (and not just the host), and these people are more mobile (and have been for months), there could be selection for reduced virulence.