As Colorado slogs through its fourth winter of the COVID-19 pandemic, a curious trend has emerged.
This season — at least so far — the state reached its peak for hospitalizations of people with COVID in the second-to-last week of November. That’s almost exactly when a peak happened last year. And the year before that. And the year before that.
Four years, four different predominant variants of the virus, four different levels of vaccination and immunity in the population. And four times that COVID hospitalizations began rising in late summer or early fall and, more notably, began to decline in Colorado right around Thanksgiving.
“Fascinating and beguiling,” is how Elizabeth Carlton, a professor of epidemiology at the Colorado School of Public Health, described the phenomenon.
“I think, by now, there probably is something happening driving this pattern,” she said — instead of the trend being a statistical fluke.
What that something is, though, no one knows.
“It is an interesting pattern,” said Dr. Rachel Herlihy, the state epidemiologist at the Colorado Department of Public Health and Environment. “I don’t think we can fully explain it.”
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What’s happening with COVID now
As it seems like everyone you know is sniffling or coughing or otherwise testing positive for something, here is what we know about Colorado’s current COVID trends:
- The late-November peak came with 280 people in the hospital with COVID.
- As of last week, there were 246 people in the hospital with COVID. (Carlton said hospitalizations have become the key metric to watch for COVID trends because they tell you how much serious illness there is; data on infections has become less reliable as more people test at home and don’t report the results.)
- After declining through December, hospitalizations began rising again after the new year, likely caused in part by the arrival of a new variant, dubbed JN.1. The variant is sweeping across the country, pushing infections higher nationally. Herlihy said the variant appears to be more transmissible and better at escaping prior immunity than others.
- It is unclear if infections are continuing to increase in Colorado, though. Up until last week, various measures used to predict the virus’ spread — things like what percentage of tests at the state’s network of “sentinel” labs are coming back positive and what percentage of emergency room visits resulted in a COVID diagnosis — were trending higher. Both of those backed off slightly last week.
- One measure used to track how widespread infections are — how much of the virus can be detected in wastewater — continues to show high levels. But both Carlton and Herlihy said it’s possible this is because JN.1 may be better at infecting cells in the gut, meaning it is shed more frequently in poop, skewing the analysis.
- The situation remains much more controlled than any previous year of the pandemic. Last year’s peak for hospitalizations was 440. The years before that saw peaks of more than 1,600 people in the hospital at one time.
Carlton said it’s important to remember that even if COVID levels are falling or aren’t as severe as in prior years, we are still in the middle of the high season for all kinds of respiratory illnesses — such as flu, RSV and that weird hacky-cough virus going around this year that no one can quite seem to identify.
That means people should continue to take precautions, such as staying home if they’re not feeling well, seeking treatment when sick and considering wearing a mask in crowded places. It’s also not too late to get vaccinated with an annual flu shot, an updated COVID booster or, if eligible, an RSV vaccine.
Colorado’s perplexing peak
Colorado’s late-November COVID peaks are unusual because they tend to happen earlier than peaks across the rest of the country. Last year, hospitalizations peaked nationally in January, same as the year before.
They have also sometimes had the effect of blunting the arrival of a new variant. When a new variant called XBB.1.5 swept across the country last year, it had little impact in Colorado, perhaps because our earlier-breaking wave had built up a layer of fresh immunity in the state.
This is not always the case. In early 2022, Colorado saw a significant rebound in infections and hospitalizations in January, driven by the arrival of the original omicron variant. This year’s trends may prove to be a smaller-scale version of that.
But the November peak has remained a COVID constant in Colorado, sticking to a much tighter turnaround schedule than other respiratory viruses like the flu, which sometimes hits early and sometimes later in winter. It is also intriguing because, with holiday travel in late November and December, one would normally expect to see cases increasing as a result.
Carlton said she’s thought of a few theories that might help explain the peaks. One possibility is weather patterns — viral transmission changes as temperature and especially humidity levels do, with drier air more favorable to infection.
“We’re just a drier state and we know this virus seems to spread under dry conditions,” she said.
The school calendar may have an impact. Colorado’s school year tends to start earlier than in some other states. Mobility and travel habits may also play a role — say, more people heading to the mountains to go skiing or spending more time indoors.
But none of these quite fits either, she said. Some falls have been wetter than others over the past four years. And, in order for hospitalizations from COVID to start falling in late November, infections would need to begin slowing at least a couple weeks prior, before ski season and school breaks really start.
Four winters in and there is still so much left to learn about this virus.
“I think what COVID has taught us is that it evolves incredibly rapidly,” Carlton said. “So what we think we know today may change by tomorrow.”