The following data is presented from David Leonhardt’s weekly Covid newsletter from the NYTs.
What explains Covid’s mysterious Two-Month Cycle? In one country after another, the number of new cases has often surged for roughly two months before starting to fall. The Delta variant, despite its intense contagiousness, has followed this pattern.
After Delta took hold last winter in India, caseloads there rose sharply for slightly more than two months before plummeting at a nearly identical rate. * In Britain, caseloads rose for almost exactly two months before peaking in July
In Indonesia, Thailand, France, Spain, and several other countries, the Delta surge also lasted somewhere between 1.5 and 2.5 months. In the U.S. states where Delta first caused caseloads to rise, the cycle already appears to be on its downside.
Case numbers in Arkansas, Florida, Louisiana, Mississippi and Missouri peaked in early or mid-August and have since been falling:
Those state-level declines are helping a nationwide flattening. Over the past week, cases have risen less than at any other point since the Delta surge began, “We still are really in the cave ages in terms of understanding how viruses emerge, how they spread, how they start and stop, why they do what they do” – Michael Osterholm, CIDRAP Director, Infectious Disease Research.
But two broad categories of explanation seem plausible: 1) The virus itself. Perhaps it spreads in waves. Some people may be especially susceptible to a variant like Delta, and once many of them have been exposed to it, the virus starts to recede — until a new variant emerges.
2) Human behavior. People live in social clusters, Jennifer Nuzzo points out. Perhaps the virus needs about two months to circulate through a typically sized cluster — and a new wave starts when people break out of their clusters, such as during a holiday…
Also: people may follow cycles of taking more and then fewer Covid precautions, depending on their level of concern, as David Dowdy and others have said.
Nothing guarantees that the Cycle will keep repeating. Maybe school openings will cause new outbreaks and extend the cycle. We just don’t know yet.
It’s clear that Covid case trends are not simply a reflection of human responses that we can track (such as mask-wearing or vaccination numbers). Viruses are more complex than that. And caseloads don’t go in only one direction. They both rise and fall, mysteriously.