Note that the following FR thread indicates that flu was “playing itself out” no later than April 10th of this year, the date of the referenced article.
US flu activity slides, death toll holds steady at 24K (April 10, 2020)
And since SARS-CoV-2 has a longer incubation / infectious period than flu, I’m not surprised that infected people carried it around for a little longer.
Insights welcome.
RE: But I’d appreciate a more scientifically-based explanation like warmer weather triggering vitamin D production or something.
Well, one way to look at it is to OBSERVE the disease progression in TROPICAL countries — places where SUMMER is practically year round -— Singapore, Malaysia, Thailand, Indonesia, Vietnam, Cambodia and even Hawaii in the USA.