Something in the way we move: The reason coronavirus came roaring back in Colorado

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In early November, as Colorado’s coronavirus case numbers surged to levels that now threaten the state’s hospital capacity and could lead to thousands more deaths before the end of the year, Gov. Jared Polis issued a plea to his constituents.

“We’ve got to live like we did in August and September. We just all eased up in October, and started taking it for granted, and got complacent,” Polis said then.

This was like the opening mystery of a pandemic potboiler: What were we collectively doing in late summer that we stopped doing in October? What’s the villain here?

Accusations flew.

It’s indoor dining at restaurants! It’s large super-spreader events!

The finger-pointing ran in circles. Experts said small household gatherings appeared to be driving the surge in cases. The New York Times published an article, citing data from Colorado, questioning whether that is the case. And the director of the federal Centers for Disease Control and Prevention rebutted the article.

But these debates missed the simpler answer, the common denominator of all the explanations for the case surge.

It’s a matter of movement.

And, by many measures, Coloradans have since the summer been moving around as much as they did pre-pandemic — maybe even more.

“The prerequisite for this current surge has been in place for a while,” said Jude Bayham, a Colorado State University economist who has been studying mobility trends to guide the state’s pandemic response.

The virus can’t jump from host to host if people aren’t moving around. So, when people move around more, there’s more potential for the virus to spread.

The rest is down to mask-wearing or distancing or hand-washing or ventilation or weather or luck. But Bayham said greater mobility plants the seeds for case surges.

Cell phone movements offer clues

Since early in the pandemic, the data company SafeGraph has been tracking something it calls its Shelter in Place Index. The company collects anonymized data on cell phone movements. The index is basically a measurement of how much of the population is staying home each day relative to a pre-pandemic baseline.

In the most recent data from Colorado, SafeGraph shows that Coloradans are back to staying home no more than normal, and sometimes less.

But there’s no single culprit driving this.

According to SafeGraph’s data, foot traffic at Colorado businesses is up since the lockdowns of the spring. But it’s still well below normal. And it has also begun to decline again in the fall as cases have risen.

Bayham also uses SafeGraph data in his research to track more specific measures. His analyses, for instance, can show how much people in individual Colorado counties are visiting restaurants, bars, grocery stores, parks or hotels. (Again, this data is anonymous and aggregated, so he and other researchers can’t use it to track any individual’s movements.)

Similar to what SafeGraph has found with visits to businesses, Bayham has found that more people in Colorado have returned to working in-person since the spring. But it’s still not back to pre-pandemic levels.

Still, Bayham has found that Coloradans are spending less time at home than they did in the spring. In many counties, the time spent at home at various points in the summer and fall has dropped below 2019 levels. Overall, Bayham said he estimates mobility in Colorado has been at 2019 levels since mid-summer.

(Bayham has produced charts for all of Colorado’s counties, available here. The charts below are for the 15 largest counties in Colorado. The dashed line shows the average amount of time people in the county spent at home per day during the stay-at-home period.)

It’s important to note that time spent outside of the home doesn’t necessarily mean people are traveling to risky places for viral spread. They could be taking a walk alone in their neighborhood or going on a hike in remote wilderness.

And exactly what people are doing when they aren’t at home differs by county. Take, for instance, the difference between Colorado’s two largest counties — Denver and El Paso.

In Denver, visits to parks have jumped in the fall, but visits to restaurants and bars have remained far below 2019 levels.

In El Paso County, home to Colorado Springs, visits to restaurants and bars increased over the summer but have since fallen off.

Fatigue and cold weather contribute to case rise

Why this pre-pandemic level of mobility is only now translating into a rise in cases is a separate mystery.

Elizabeth Carlton, an infectious disease researcher at the Colorado School of Public Health who works with Bayham as part of the modeling team helping the state project the pandemic’s course, said COVID fatigue likely plays a big role. People just want to go back to their normal lives, which leads them to let down their precautions in what they consider “safe” environments, like their own home in the company of friends.

“This virus is exhausting,” Carlton said. “And disruptive. And causing us to rethink everything. To be safe, we have to change how we worship, how we eat, how we mix with our family and friends. And that’s a big ask.”

The weather also likely plays a role. Colder weather pushes personal gatherings from well-ventilated back patios to poorly ventilated indoor spaces. But the colder temperatures also likely help the virus remain viable for longer outside the human body, giving it a greater chance to infect someone new.

“It may be,” Carlton said, “that we need to work harder now to control the virus than we did during the summer because the weather is making things less favorable for us.”

The state has tried to do that hard work by tightening restrictions on restaurants and gatherings. But, in a briefing to Colorado reporters on Tuesday alongside Polis, Dr. Anthony Fauci, the nation’s top infectious disease expert, said Thanksgiving-related travel may mean Colorado, like the rest of the nation, is poised for “a surge upon a surge” of cases and hospitalizations that could begin hitting in two to three weeks.

The nation, Fauci said, is poised for a month of “precarious risk.” And that risk will come in all sorts of places — at work, at the store, in gatherings with friends or family. People can mitigate the risk, he said, by wearing masks. Maintaining their distance. Meeting with people outside rather than inside. Washing their hands.

But the most effective strategy is both the simplest and, as we enter the 10th month of the pandemic, perhaps the hardest. It addresses the root cause of almost all new cases but also asks the most of us.

Think about your mobility footprint. As much as possible, just stay home.

“Avoid the things that we know are pleasant and desirable though they’re dangerous now,” Fauci said. “… It’s not too late to do something about it.”