The increase in new daily Covid-19 cases has hit its lowest point since June—leading some epidemiologists to speculate that, because Covid-19 surges tend to follow a "mysterious" two-month cycle, the United States' delta-driven surge may have peaked, David Leonhardt and Ashley Wu report for the New York Times.

Two-month patterns

Since the start of the pandemic, Covid-19 surges have tended to follow a "regular—if mysterious"—pattern, with cases rising for two months before they start to fall, Leonhardt and Wu report. This was the case with the alpha variant from earlier this year, and surges driven by the delta variant in other countries appear to have followed the same pattern.

For example, in India, Covid-19 cases surged for just over two months before dropping at a sharp rate. And in the United Kingdom, cases rose for two months before hitting their peak in July.

Indonesia, Thailand, France, Spain, and many other countries have seen similar patterns with the delta variant where cases rise for 1.5 to 2.5 months and then drop, Leonhardt and Wu report.

Some states in the United States have started to see this pattern with delta. Covid-19 case rates in Arkansas, Florida, Louisiana, Mississippi, and Missouri peaked in early or mid-August and now appear to be falling, Leonhardt and Wu report.

Viral or human behavior?

Experts aren't quite sure why the coronavirus surges have often followed these two-month patterns. "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," said Michael Osterholm, an epidemiologist at the University of Minnesota.

Experts offered two theories as to why this pattern could be happening. First, it could be related to the virus itself. The virus might spread in waves, as certain people could be more susceptible to variants than others. Once the virus infects them, it could start to recede until a new variant arrives or the population approaches herd immunity, Leonhardt and Wu report.

The other theory is related to people's behavior. According to Jennifer Nuzzo, an epidemiologist at Johns Hopkins University, people live in social clusters, and it's possible the virus needs around two months to circulate through an average-sized cluster. Then, new waves start when people break out of their social clusters, like during a holiday.

Waves could also follow how strictly people adhere to coronavirus precautions, Leonhardt and Wu report.

Has the delta surge peaked in the US?

There have been exceptions to the two-month pattern, Leonhardt and Wu report. Brazil, for example, hasn't seen any pattern to its Covid-19 surges. And while cases in the United Kingdom declined around two months after its delta peak, they started rising again after a couple of weeks, likely due to the removal of pandemic precautions.

It's possible the start of the school year in the United States could trigger another surge in Covid-19 cases, Leonhardt and Wu report.

The key to avoiding another surge is "vaccine, vaccine, vaccine," Osterholm said. Some countries, including Malta and Singapore, have seen vaccination rates rise enough to reduce the two-month pattern of Covid-19 surges to two weeks.

"Our top goal has to be first shots in arms," Nuzzo said. (Leonhardt/Wu, New York Times, 9/1)