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 hospitalised with COVID has fallen about 25% since Sept. 1. Daily deaths — which typically change direction a few weeks after cases and hospitalisations — 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 recognise: 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 behaviour 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 behaviour 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.

Whatever the reasons, the two-month cycle keeps happening. It is visible in the global numbers: Cases worldwide rose from late February to late April, then fell until late June, rose again until late August and have been falling since.

The pattern has also been evident within countries, including India, Indonesia, Thailand, Britain, France and Spain. In each of them, the delta variant led to a surge in cases lasting somewhere from 1 1/2 to 2 1/2 months.

In the US, the delta surge started in several Southern states in June and began receding in those states in August. In much of the rest of the US, it began in July, and cases have begun falling the past few weeks. Even pediatric cases are falling, despite the lack of vaccine authorisation for children younger than 12, as Jennifer Nuzzo of Johns Hopkins University told The Washington Post.

I want to emphasise that these declines may not persist. COVID’s two-month cycle is not some kind of iron law of science. There have been plenty of exceptions.

In Britain, for example, caseloads have seesawed over the past two months, rather than consistently fallen. In the US, the onset of cold weather and the increase in indoor activities — or some other unknown factor — could potentially spark a rise in cases this fall. The course of the pandemic remains highly uncertain.

But this uncertainty also means that the near future could also prove to be more encouraging than we expect. And there are some legitimate reasons for COVID optimism.

The share of Americans ages 12 and older who have received at least one vaccine shot has reached 76%, and the growing number of vaccine mandates — along with the likely approval of the Pfizer vaccine for children ages 5 to 11 — will increase the number of vaccinations this fall. Almost as important, something like one-half of Americans have probably had the COVID virus already, giving them some natural immunity.

Eventually, immunity will become widespread enough that another wave as large and damaging as the delta wave will not be possible. “Barring something unexpected,” said Dr. Scott Gottleib, a former Food and Drug Administration commissioner and author of “Uncontrolled Spread,” a new book on COVID, “I’m of the opinion that this is the last major wave of infection.”

COVID has not only been one of the worst pandemics in modern times. It has been an unnecessarily terrible pandemic. Of the more than 700,000 Americans who have died from it, nearly 200,000 probably could have been saved if they had chosen to take a vaccine. That is a national tragedy.

COVID also isn’t going to disappear anytime soon. It will continue to circulate for years, many scientists believe. But the vaccines can transform COVID into a manageable disease, not so different from a flu or common cold. In the past few weeks, the country appears to have moved closer to that less-grim future.

Whatever the next few months bring, the worst of the pandemic is almost certainly behind us.