New cases in the U.S. have fallen by more than a third in the past month.
By David Leonhardt
That Two Month Cycle
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 U.S. has fallen 35 percent since Sept. 1:
Worldwide, cases have also dropped more than 30 percent since late August. “This is as good as the world has looked in many months,” Dr. Eric Topol of Scripps Research wrote last week.
These declines are consistent with a pattern that regular readers of this newsletter 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, like Delta — and then declined for about two months.
Epidemiologists do not understand why. Many popular explanations, like seasonality or the ebbs and flows of 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.
Human behavior does play a role, with people often becoming more careful once caseloads begin to rise. But social distancing is not as important as public discussion of the virus often imagines. “We’ve ascribed far too much human authority over the virus,” as Michael Osterholm, an infectious-disease expert at the University of Minnesota, has told me.
The recent declines, for example, have occurred even as millions of American children have again crowded into school buildings.