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This scanning electron microscope image shows particles of the virus SARS-CoV-2 (round magenta objects) emerging from the surface of cells cultured in the lab. SARS-CoV-2 is the scientific name of the virus that causes COVID-19. The virus shown was isolated from a patient in the United States. (Provided by the National Institute of Allergy and Infectious Diseases's Rocky Mountain Laboratories in Montana)

Infections from a dangerous new coronavirus variant are surging across Colorado, causing health officials to amp up the urgency in their messages for people to get vaccinated.

The Delta variant, which is believed to have originated in India and is also known as the B.1.617.2 variant, was first identified in Colorado in late April. By the first week of this month — a period of about six weeks — it had grown to account for an estimated 40% of all new infections in Colorado.


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.


For comparison, it took eight weeks for the Alpha variant — also known as the B.1.1.7 or United Kingdom variant — to reach 40% of the state’s total cases. As of early June, that variant made up an estimated 48% of total cases in Colorado, but its share is falling as Delta’s rises.

Nationwide, the Delta variant is believed to account for about 10% of new cases.

Dr. Rachel Herlihy, the state epidemiologist, said the Delta variant is believed to be about 50% more transmissible than the Alpha variant, which itself was 50% more transmissible than the original form of the virus.

“At this point, it is the most transmissible variant we’ve seen,” Herlihy said.

But, for most people who have been fully vaccinated, this news shouldn’t cause too much alarm. While the vaccines are believed to be slightly less effective against the Delta variant compared with other forms of the virus, they still offer strong protection.

A study in the United Kingdom, for instance, found that two doses of the Pfizer vaccine are 96% effective against the Delta variant at preventing severe COVID-19 requiring hospitalization.

For unvaccinated people, it’s a different story.

The high transmissibility of the variant means it is less likely people will be able to avoid the virus without protection from a vaccine, Herlihy said. The variant is just too good at jumping from vulnerable person to vulnerable person.

And that means, even as the state opens back up and life returns to its pre-pandemic norms, there is more danger now to people who aren’t vaccinated.

“I would say, at this point in the pandemic, the risk to unvaccinated individuals is greater than it was several months ago,” Herlihy said.

U.S. Health and Human Services Secretary Xavier Becerra, who was in Colorado last week to meet with Gov. Jared Polis and tour a vaccine clinic, said that the Delta variant shows why it’s important to be fully immunized against COVID-19.

“When you only have one dose, you’re not fully protected,” Becerra said. “It is really important that folks make sure they follow through and get that second shot. We’re seeing that some of these variants are tough.”

The Delta variant has been found in 24 counties scattered across the state. It is most prevalent in Mesa County, where the variant was first discovered in the state and where at least 248 cases have now been identified through a statewide surveillance system.

Positive coronavirus tests must be genetically sequenced to determine which variant caused the infection. Because the surveillance system collects only a small number of tests each week to sample, there are likely many more cases than the state has identified. Herlihy said it is believed that almost all new cases in Mesa County are being caused by the Delta variant.

Mesa County earlier this month issued a public health alert warning residents that the county was seeing “widespread transmission” of the variant. The alert said the age group with the most infections over the previous two weeks was kids ages 10 to 19. Children ages 9 and under had also seen a high number of infections.

Mesa County now has one of the highest case rates in the state, and its hospitals are showing signs of strain. On Thursday, 90% of the county’s intensive-care hospital beds were in use — though that figure encompasses coronavirus patients and people in the hospital for other reasons.

The county also has a below-average vaccination rate. As of Thursday, about 40% of the county’s eligible population had been fully vaccinated. The statewide average is about 57%.

To Herlihy, these two data points are potentially linked. The state has previously shown how counties with lower vaccination rates have higher hospitalization rates. That the Delta variant took off in Mesa County only underscores the threat it poses to unvaccinated people.

As a result, the Colorado Department of Public Health and Environment is stepping up its vaccination efforts in the county. The department is deploying mobile vaccination clinics in multiple locations in Mesa County over the next week.

It also plans to have a vaccination bus at the Country Jam festival June 24-26 at a ranch west of Grand Junction. (The festival is also doing its part, with a note on its website warning: “Mesa County is currently experiencing some of the highest COVID rates in the country. In preparation for your visit to Country Jam, we strongly urge you to get vaccinated.”)

The state is stepping up its vaccination campaign in other ways, too. Starting this week, CDPHE will begin making cold calls to adults in Colorado who have not yet been vaccinated. Callers will provide information about vaccination and will offer to help schedule appointments. The department will also start sending text messages to people who may be overdue for their second vaccine dose.

“With or without the Delta variant,” Herlihy said, “vaccination against COVID-19 continues to be the best way to protect yourself.”

Staff writer Jesse Paul contributed to this report.

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...