Pandemic predictions: Two-month cycle and immunity effect

October 14, 2021
Graph showing peaks of COVID in Tricounty area.
“It just seems hard to believe we wouldn’t have another wave," says Dr. Michael Sweat, leader of MUSC's COVID tracking team.

The pandemic that seemed so wildly unpredictable at first has settled into a pattern. And this week’s 28% decrease in COVID cases in the Tri-county area appears to be part of it.

“There's this weird two-month thing that goes on. We all picked it up," said Michael Sweat, Ph.D., leader of the Medical University of South Carolina’s COVID-19 tracking team, referring to his fellow scientists. "I mean, it's like every time when you get a wave, it goes up, peaks and declines in about two months,”

You can see it in his team’s graph tracking the trajectory of COVID cases in the Charleston area. The first wave began in June 2020 and came down in August. The second significant wave started in November and began to fall in January.

Meanwhile, people started getting vaccinated. Case numbers plunged. Then, in July 2021, the Delta variant began its drive toward a new pandemic high. It peaked and declined within two months.

“There's a lot of questions in the epidemiology world around what's driving the two-month cycle. Is it something innate to the viral situation? I think it may be people changing behavior, and maybe it runs through the people most at risk at that point in time,” Sweat said.

Dr. Michael Sweat 
Dr. Michael Sweat

Unfortunately, he predicts it could happen again this winter. “It just seems hard to believe we wouldn’t have another wave. The signals from other places aren’t very reassuring. In the U.K., approximately 93% of people have either been infected or vaccinated. And yet they're seeing these resurgences,” Sweat said. “I think we'll get to an endemic situation, but I just don't think we're there yet.”

But we are at a point where the percentage of people diagnosed with COVID who end up in the hospital is declining, Sweat said. “I’m surprised we’re not hearing more about this. It was around 25% in the first wave, then in the winter wave it was around 15% — maybe lower. And then in this current wave, we’re down here around 10%. It’s all suggesting that immunity is starting to have an effect.”

That could bode well for a potential winter wave. For now, Sweat is glad to see COVID case numbers falling. For the week of Oct. 5 through 11, there were about 1,400 COVID cases in Berkeley, Charleston and Dorchester counties combined, compared with almost 2,000 the week before.

The COVID tracking team also lowered COVID’s estimated impact on the Charleston area from “severe” to “significant,” because the number of reported cases per day per 100,000 people fell to 25. That’s the lowest it’s been since late July.

But Sweat, a professor in the College of Medicine at MUSC who’s also affiliated with the Johns Hopkins Bloomberg School of Public Health, warned against what he called magical thinking. “When it gets better, you have this very optimistic bias saying, ‘Oh, it's going away.’ I just worry that people see these numbers come down and throw caution to the wind. It's dangerous because it's what frequently causes those surges.”

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