Thread by @bansisharma

1. Covid's Mysterious Two Month Cycle
Since the pandemic began, Covid has often followed a regular — if mysterious — cycle. In one country after another, the number of new cases has often surged for roughly two months before starting to fall.

2. The Delta variant, despite its intense contagiousness, has also followed this two-month 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.

3. 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.

4. And 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:

5. Experts cannot explain it. “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, an epidemiologist at the University of Minnesota, said.

6. Whatever the reasons, the two-month cycle predated Delta. It has repeated itself several times in the U.S., including both last year and early this year, with the Alpha variant, which was centered in the upper Midwest.

7. There have been plenty of exceptions to the two-month cycle around the world. In Brazil, caseloads have followed no evident pattern. In Britain, cases did decline about two months after the Delta peak — but only for a couple of weeks. Cases there have been rising again.

8. In the U.S., the start of the school year could similarly spark outbreaks this month. The country will need to wait a few more weeks to know. In the meantime, one strategy continues to be more effective than any other in beating back the pandemic: “Vaccine, vaccine, vaccine.”

9. The vaccine is so powerful because it keeps deaths and hospitalizations rare even during surges in caseloads. In Britain, the recent death count has been less than one-tenth what it was in January.

10. In a few countries, vaccination rates have apparently risen high enough to break Covid’s usual two-month cycle: The virus evidently cannot find enough new people to infect. In both Malta and Singapore, this summer’s surge lasted only about two weeks before receding.

The End