Image used for illustration purpose only.

Cory Franklin and Robert A. Weinstein, Tribune News Service

The Greek historian Herodotus lived through the plague of Athens, one of the world’s first great pandemics. He wrote, “Circumstances rule men; men do not rule circumstances.”

So it is with COVID-19. In the US, we are currently in the middle of an autumn downturn in COVID-19 cases, and no one can say whether this will be the end. It is clear, however, that the US is repeating a mystifying cycle of case rise and fall that has been seen in other countries. For reasons unknown, cases surge for six to 10 weeks and then fall predictably in a similar fashion for at least an equal period.

The current wave in the US began in Missouri/Arkansas in the last week of June with cases rising nationally from 13,000/day to peaking at nearly 170,000/day in early September. Case numbers have been falling since then to the current level of more than 65,000 per day. At present, we are nearing the eighth week of the downturn in cases.

With luck, if the trends hold, cases are likely to fall for at least another one to three weeks. After that, no one can say what will happen. A new variant, delta-plus, has emerged sporadically, which might complicate any late-year predictions.

A similar pattern of a roughly eight-week cycle was seen this spring in India, which had the highest peak of any country in the world. The same cycle played out in Indonesia in early summer, and in Japan in the lead-up to the Olympics.

Not all countries behave in this fashion; the UK, Israel and South Korea have demonstrated somewhat different curves.

But a look at worldwide case totals, the sum of the cases in every country that smooths out variations, clearly demonstrates curves consistent with the six-to-10 week cycle of rise and fall.

Why does COVID-19 behave like this? In an admission of humility somewhat surprising for COVID-19 experts, prominent US public health researcher Michael Osterholm conceded, “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.”

Because rises in cases are easier to explain, they often draw more attention than case declines.

The six-to 10-week interval may be the time it takes for spread to susceptible hosts. Especially with contagious variants, dramatic surges may be fueled by 20% of infected patients — the so-called superspreaders, Nature.com suggests. Subsequent infections in the 80% of the population who are less apt to spread infection are likely to trigger greater precautions (masking, lockdowns, distancing). That, along with fewer susceptible hosts, may stop the spread — until a new cycle begins.

But peaks at six to 10 weeks followed by declines cannot be explained completely by restrictions or masking. Often, these factors are put into place even as the declines are beginning.

A common error is mistaking correlation, when two events happen simultaneously, for causation, when one causes the other. Lockdowns in non-island countries have not been especially effective in halting the spread of COVID-19 except as short-term “circuit breakers.”

Greater attention to case declines might yield more answers. Dramatic case declines as occurred in Japan (97% in nine weeks) and Indonesia (95% in 10 weeks) have left the experts flummoxed. Vaccination, masking and social distancing all played a factor in the downturns, but are almost certainly not the primary reason.

Japan was relatively late to vaccination and never instituted a full lockdown. Kenji Shibuya, a Japanese epidemiology expert, admitted as much: “Season factor, human factor and viral factor: I think that seems to be a very complicated interactional role.” Earlier this year, India went from a peak of 400,000 daily cases to 40,000 daily cases in eight weeks, despite having a low vaccination rate and minimal social distancing.

Why would social behavior, so variable in different countries, result in such similar cycles of infection? We are left with two other factors, both of which we know very little about — the virus itself and host immunity.

Does the virus have a built-in regulator that governs its behavior and limits it to finite cycles of infection before it retools? Can the virus, like Dracula, shut itself off at two-month intervals and reemerge in slightly different forms?

Are there different patterns of host immunity, natural and acquired, in different countries? Does immunity explain the lower rate of infection and mortality in East Asia and Africa compared to Europe and the Western Hemisphere? Is that immunity a driver in some way of the cyclical rise and fall of cases? The medical textbooks of the next hundred years, or whatever passes for textbooks in the future, will note that the COVID-19 pandemic is the first time scientists finally have the technical resources and opportunity to investigate the crucial issue of pandemic immunity.

This will be a key new area of research borne of the pandemic.