The heretofore common coronavirus strains were fairly seasonal, at least in the temperate latitudes where most studies have been done. Infections dropped off dramatically beginning in June and continuing until October. No one really understands why-the virus doesn’t like warm and humid weather, vitamin D levels, we are indoors/outdoors more, ultraviolet levels, etc. have been advanced as explanations. And this strain definitely is operating on a weird pattern. The northeast’s drop in cases I understand, they had such bad infection levels near the start that if you buy the T cell argument, they probably have enough population immunity to slow spread and they finally stopped the stupid nursing home policies. But why are some southern and southwestern states seeing more cases now; they are plenty hot and humid, their residents get more sunshine and should have more vitamin D year round, they get plenty of UV. Some have suggested that influenza has a different seasonal pattern in southern states–it has a relatively higher prevalence in the summer, so maybe this coronavirus is like that. And people in Minnesota spend plenty of time in the air-conditioned indoors. Protests might have something to do with it, but then we should see spikes in a lot of states. Reopenings can’t have much to do with it, many of the states were open for weeks before cases started rising and Wisconsin hasn’t had a statewide order in place since May 13, and has seen no significant rise. Texas, California and Arizona may be seeing some border contribution and Florida claims out-of-staters represent many of the cases. It seems like yet another mystery of the coronavirus universe.
One thing is clear, deaths and hospitalizations have fallen pretty dramatically in Minnesota. Hospitalizations have declined from a peak of around 800 patients in a day to about 250 now. Since June 21, a total of 15 reporting days, there have been ten or more deaths on only two days. In the prior 15 days, there were less than ten deaths only twice. Since long-term care residents continue to represent around 80% of deaths, not sure that we have cleaned up our act in that sector. Leaving aside the last few days where data lags play a role, the number of cases shows no discernible trend. Our positive rate has gone down. It sure looks like an epidemic that has petered out. I don’t think our rate of social distancing, mobility or mask use is any different than many other states. Most likely explanations are some form of seasonality or enough population immunity to dramatically slow spread, through a combination of low susceptibility due to pre-existing immune defenses, and higher prevalence of infections by this strain than we currently realize.
Wisconsin is more of the same, with a continuing much lower death rate than Minnesota. That state has only seen 58 deaths in the last 15 reporting days and the rate of growth in newly hospitalized patients has slowed to a crawl, while testing has stayed at a fairly high level. I am sure at some point an answer will become apparent, but it isn’t right now.