COVID's Mysterious 2-Month Cycle --Trying to Understand it

COVID-19 is once again in retreat.

The reasons remain somewhat unclear, and there is no guarantee that the decline in caseloads will continue. But the turnaround is now large enough — and been going on long enough — to deserve attention.

The number of new daily cases in the United States has fallen 35% since Sept. 1. Worldwide, cases have also dropped more than 30% since late August. “This is as good as the world has looked in many months,” Dr. Eric Topol of Scripps Research wrote last week.

The most encouraging news is that the most serious forms of COVID are also declining. The number of Americans hospitalized with COVID has fallen about 25% since Sept. 1. Daily deaths — which typically change direction a few weeks after cases and hospitalizations — have fallen 10% since Sept. 20. It is the first sustained decline in deaths since early summer.

These declines are consistent with a pattern that readers will recognize: COVID’s mysterious two-month cycle. Since the COVID virus began spreading in late 2019, cases have often surged for about two months — sometimes because of a variant, such as delta — and then declined for about two months.

Public health researchers do not understand why. Many popular explanations — such as seasonality or the ebbs and flows of mask wearing and social distancing — are clearly insufficient, if not wrong. The two-month cycle has occurred during different seasons of the year and occurred even when human behavior was not changing in obvious ways.

The most-plausible explanations involve some combination of virus biology and social networks. Perhaps each virus variant is especially likely to infect some people but not others — and once many of the most vulnerable have been exposed, the virus recedes. And perhaps a variant needs about two months to circulate through an average-sized community. ...