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This image shows a transmission electron micrograph of SARS-CoV-2 virus particles (in gold) within endosomes of a heavily infected nasal olfactory epithelial cell. The image was captured at the National Institute of Allergy and Infectious Diseases Integrated Research Facility in Fort Detrick, Maryland. (Provided by NIAID)

To understand one of the essential mysteries of how and when the coronavirus spreads in Colorado, you need look no further than a basic graph of case trends.

Nearly two years into the pandemic, an astonishing symmetry is contained within it.

This graph from the Colorado Department of Public Health and Environment shows the moving 7-day average number of daily new coronavirus cases reported, as of Dec. 15, 2021. (Provided by CDPHE)

Colorado has had five distinct waves of cases, but the two biggest are the ones to focus on here. Both surged during the waning days of summer, last year and this year. Both barreled through Halloween like runaway freight trains, worrying public health officials that the surges might become supercharged by coming holiday gatherings.

Instead, both waves began receding around mid-November, and, despite small spikes following Thanksgiving, they kept dropping. On Wednesday, the 7-day average of new cases being reported each day in Colorado dropped to its lowest since September.

Hospitalizations for COVID-19 have followed a similar pattern. They peaked last year Dec. 1. They peaked this year Nov. 23.

“I think it’s a really interesting observation that, almost to the day, the trends are following each other from last year compared to this year,” said Dr. Rachel Herlihy, the state epidemiologist.

What makes the mystery so intriguing is that it defies simple explanation.

Maybe it has to do with vaccination? But cases last year began falling before vaccines were rolled out across the state, and cases began falling this year before booster shots were widely approved.

Something to do with school schedules? As Dr. Jonathan Samet, the dean of the Colorado School of Public Health, pointed out, not all schools had in-person learning last fall, while they did this year.

Mask rules and other restrictions? The state and counties implemented a series of increasing restrictions last year to stop the rise in cases, but the most dramatic steps taken this year – metro-area counties reimposing mask orders – took place after reports of new cases began falling. (Metro-area health officials said the orders were still necessary in order to protect hospital capacity.)

What about seasonal weather patterns? But November this year was one of the warmest and driest on record in many parts of the state, while last year saw only slightly above-average temperatures and below-average precipitation.

Mobility patterns? Nope.

“If you look at the activity level and the mixing, the cellphone data, certainly this year we were much more active and engaged and mixing and mobile,” Samet said.

Dr. Jonathan Samet, the dean of the Colorado School of Public Health, speaks during a July 9, 2020, news conference with Gov. Jared Polis at the governor’s mansion. (Jesse Paul, The Colorado Sun)

Ask epidemiologists like Samet and Herlihy what they think is going on here, and over and over you will hear the same thing.

“I think a lot of questions still remain about what precisely drives these waves of illness,” Herlihy said.

“I can’t pinpoint why that happened,” Samet said.

But what experts also agree on is, while one single factor probably isn’t responsible for the seasonal symmetry, that doesn’t mean that it doesn’t play a role.

Take weather, for instance. It’s often cited as one of the most likely culprits – both because of the way it might affect the virus and because of the way it influences human behavior.

A large Yale School of Public Health study published this year found that the SARS-CoV-2 coronavirus fares better in colder weather, in drier weather and in weather where there is less ultraviolet radiation. So, basically, winter.

But, even more interesting, the study tried to determine the relative impact each of those meteorological factors played in transmission. And temperature came in last. The research team concluded that humidity played the biggest role in transmission – accounting for more than 9% of a basic measurement for the virus’s transmissibility.

This might explain why cases in the U.S. West all surged at the same time this fall, while cases across other – typically wetter – parts of the country fell.

Samet said colder weather also pushed people into close, indoor places more favorable to transmission. And, though fall this year was warmer than last, he said relatively small temperature differences might not matter as much as the general trend. In July, you can eat dinner on a restaurant patio. In November, you likely can’t.

State epidemiologist Rachel Herlihy speaks during a news conference on the state’s response to the COVID-19 pandemic, Tuesday, Dec. 7, 2021, in Denver. (AP Photo/David Zalubowski)

There’s another aspect of human behavior that’s harder to quantify: How much people care about case trends. In both this year and last, state and local health leaders filled the weeks prior to Thanksgiving with a steady drumbeat of cautionary messages. They became increasingly vocal warning the public about the rise in cases.

Eventually, Samet said, that breaks through.

“I do think there’s a tendency for people, as the curve goes up, to begin to change their behavior and perhaps be more cautious,” he said.

But, overall, he and Herlihy said this mystery – so important to gaining better control over the virus that has upended everyone’s lives – probably won’t be solved soon.

“I think it’s a question that scientists are going to be studying for many years to come,” Herlihy said.

John Ingold is a co-founder of The Colorado Sun and a reporter currently specializing in health care coverage. Born and raised in Colorado Springs, John spent 18 years working at The Denver Post. Prior to that, he held internships at...