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A coronavirus piñata at a vaccine event in Henderson on Feb. 26, 2021. (Jesse Paul, The Colorado Sun)

Don’t get too cocky, Colorado, but we have some eagerly awaited news for you: Parts of the state are slowly, but steadily, inching closer to herd immunity from coronavirus — one way or another.


The latest from the coronavirus outbreak in Colorado:

  • MAP: Cases and deaths in Colorado.
  • TESTINGHere’s where to find a community testing site. The state is now encouraging anyone with symptoms to get tested.
  • VACCINE HOTLINE: Get up-to-date information.


More than 50% of the state’s total population has now been fully immunized against the virus, according to figures from the Colorado Department of Public Health and Environment. (The state was one of only 20 to reach President Joe Biden’s goal of having 70% of its adult population receive at least one dose of vaccine by July 4.)

But, when you add in immunity that comes from having been infected by the virus, there are regions of the state where likely more than 60% of the population is currently immune, according to modeling estimates. Herd immunity — the level at which enough people are immune that the virus is forced into decline — has often been pegged at around 70% for coronavirus, though that number may be changing as more transmissible variants take over.

Where the most-immune regions are and how they achieved their immunity, though, says a lot about the state of the pandemic in Colorado right now. Here’s what you need to know.

There are two routes to herd immunity

There’s more than one way to get to herd immunity. Both vaccination and infection get the job done — though, of course, the latter route is rougher.

Every few weeks, the Colorado COVID-19 Modeling Group, which is made up of university researchers around the state, releases a region-by-region report on the state of the pandemic. Among the figures it produces is an estimate for coronavirus immunity, which takes into account the vaccination rate in the region and the estimated infection rate.

In its latest report, the modeling team estimated that two regions in Colorado have overall immunity percentages above 55%: The East Central region, which includes Cheyenne, Elbert, Kit Carson and Lincoln counties; and the metro area, including Gilpin and Clear Creek counties.

Two more regions — the Northeast region and the South Central region — were above 50%. And, because the report’s estimates only considered data through the middle of June, those numbers are all higher now — likely above 60% in the most-immune places.

(The team produces estimates by region because it helps smooth over data blips from small population sizes in some counties and because the regions better encapsulate how people actually move around in the state — the bubbles where people live and shop and mingle.)

Interestingly, the East Central and the metro area came about their immunity differently. The modeling team’s report estimated that the metro area had one of the highest vaccination rates in the state — around 55% of the population was fully vaccinated, as of late June. The East Central region, meanwhile, had the lowest vaccination rate — only about 26% were fully vaccinated. Instead, the East Central region’s estimated immunity comes largely through infection, according to the report.

Boulder resident Ruth Wight drove to Limon and stayed at a motel about 15 miles from the Lincoln County Fairgrounds in Hugo to make sure she got a vaccination on March 19, 2021. “I feel like I could cry. I am so happy,” she said after receiving her dose of the Johnson & Johnson coronavirus vaccine. (Brian Malone, Special to The Colorado Sun)

Colorado is unevenly vaccinated

This insight into the differences among highly immune regions of the state underlines a bigger issue: Vaccinations rates are extraordinarily uneven across Colorado.

As of Tuesday, CDPHE reported that more than 70% of the eligible population has been fully vaccinated against coronavirus in six counties. Another 24 counties have at least 50% of their eligible population fully vaccinated.

Meanwhile, fewer than 40% of the eligible population has been fully vaccinated in 18 counties, including four counties where less than 30% of the population has been fully vaccinated.

This means the pandemic is fracturing into multiple, smaller epidemics

Where large pockets of unvaccinated people exist, the virus has the chance to thrive. That will lead to an increasing unevenness in how the pandemic is experienced across the state.

“We’re going to see a lot of patchiness in who’s affected in where there are problems,” said Dr. Jonathan Samet, the dean of the Colorado School of Public Health.

A good example of this right now is in Mesa County, where the vaccination rate is low. Coronavirus cases, driven by the highly transmissible Delta variant of the virus, have swamped local hospitals. Entering the Fourth of July weekend, 96% of the region’s hospital beds were occupied. In the Denver metro area, where vaccination rates are higher, about 80% of hospital beds were occupied.

Samet said this shows how the coronavirus pandemic is no longer a single thing in Colorado. It will take different paths in different communities based on vaccination rates.

“We really have a series of epidemics now,” Samet said.

This chart, produced by the Colorado Department of Public Health and Environment, shows a rolling 7-day average of new coronavirus cases. New infections have declined in Colorado since May, but the decrease has stalled. (Provided by CDPHE)

Overall, things are improving in Colorado

Despite these hot spots across the state, infections overall are in decline across the state.

Between 300 and 350 new cases are being reported per day, on average. That’s down from more than 1,500 cases per day in late April.

The decline in cases has leveled off, though. The total number of new cases per week has been increasing slightly for the past two weeks.

Around 280 people are currently hospitalized in Colorado with confirmed coronavirus infections — down from nearly 700 in early May. But hospitalization numbers have also flattened out in recent weeks.

About four or five people with coronavirus are dying per day in Colorado.

“Overall things are moving in the right direction, albeit slower than I think we all wish as a consequence of the Delta variant,” Samet said.

He added: “What you might say is we could have declined faster if we had not had this more transmissible strain.”

But the virus is still spreading more now than it was last summer

This improvement shouldn’t mask the fact that the coronavirus is more prevalent in Colorado now than it was last summer.

At this time last year, the state was seeing about 280 new coronavirus cases per day, and about 170 people were in the hospital with confirmed coronavirus infections. 

Nationally, Colorado is ranked eighth among U.S. states for the highest new coronavirus case rates — an improvement from May, when the state was ranked No. 1, but still nothing to celebrate.

This is partly due to the Delta variant, which has hit Colorado harder so far than it has hit most other states. The variant is estimated to account for 80% of all new coronavirus cases in the state, compared with about 20% of new cases nationally.

But Samet said the lower case rates at this time last year were also due to the prolonged statewide shutdown last spring. And, he said, people were likely more cautious last year, too.

“People are behaving like it’s 2019 all over again,” he said. “It’s not.”

Patrons enjoy food and drinks at Tom’s Watch Bar at Coors Field on Saturday, June 26, 2021 in Denver. (Olivia Sun, The Colorado Sun)

A post-Fourth of July surge is unlikely

One advantage we have this year over last: Samet said it is unlikely we will see the same kind of post-Fourth of July case wave that we saw in 2020.

Why? Because last year’s wave — the state’s second of what have now been four distinct waves of the virus — was caused by people breaking their isolation and gathering around the July 4 holiday. But Samet noted that in 2021 we’ve been gathering with abandon for months now.

Bars and restaurants are operating at full indoor capacity. The Rockies can host full-capacity games. Mask orders have gone away. And cases continue to decline overall.

“We’re sustaining all of that,” Samet said.

But the variants raise the bar for declaring victory

Just as Colorado starts creeping up toward herd immunity, though, the goal posts have moved.

Blame the Delta variant — as well as the Alpha variant, which is also more transmissible than the original form of the coronavirus and accounts for an estimated 14% of new cases in the state. These more-transmissible forms mean that the herd immunity threshold has risen — perhaps to around 80%, Samet said.

That’s because herd immunity comes from a relatively simple calculation, one where the only variable is how transmissible the virus is. The more transmissible the virus, the higher the percentage needed for herd immunity.

And, to Samet, the best way to continue boosting those immunity numbers in Colorado is through vaccination.

“The more people who are vaccinated,” he said, “the better off we are as a society.”

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...