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This electron micrograph image, which has been artificially colorized, shows monkeypox virus particles, in orange, purified from a cell culture at the National Institute of Allergy and Infections Disease's Integrated Research Facility in Fort Detrick, Maryland. (Provided by NIAID)

It appears the monkeypox virus may be gaining a foothold in Colorado, prompting health officials to offer vaccination to people in high-risk groups.

On Wednesday, state epidemiologist Dr. Rachel Herlihy said Colorado has seen a potential change in transmission patterns for recent monkeypox cases and the virus could be spreading undetected. As of Thursday afternoon, Colorado has seen eight cases of monkeypox — including two found in the last week.

“What we have seen in some recent cases is more clear evidence of possible community transmission,” she said.

Up until now, Colorado’s monkeypox cases have been tied to either travel to an area with an outbreak or contact with someone in the state who was known to have the virus. But Herlihy said one recent case in the state had neither of those factors. There are two more recent cases where it is also unclear how the person contracted the virus.

“That’s why we want to be proactive,” she said.

Expanding vaccine eligibility

Monkeypox, an orthopoxvirus that is a less-serious cousin of smallpox, can be caught by anyone who has close contact with someone who is infected. Herlihy said most of the cases in the recent outbreaks in Colorado and in other countries are in men who have sex with men.

As a result, the Colorado Department of Public Health and Environment scheduled two vaccine clinics this week — one Thursday and one Friday.

“We can’t sit on this valuable tool we have to prevent spread,” Scott Bookman, the director of CDPHE’s Disease Control and Public Health Response Division, said in a statement.

The clinics were open to men 18 and older who are gay, bisexual or have sex with men. To be eligible, men also had to have had anonymous sex partners or multiple sex partners in the past 14 days.

Colorado had 200 doses of vaccine to administer at the clinics, and, by Thursday afternoon, all available appointments were booked. Herlihy said the state expects to host more vaccine clinics in the future. In the meantime, CDPHE is encouraging anyone who believes they have monkeypox or were exposed to the virus to contact a health care provider as soon as possible, get tested and avoid contact with others.

A change in federal response

The clinics are possible because the federal government this week widened access to the vaccine and pushed more doses out to the states, hoping to slow spreading monkeypox outbreaks. The vaccine being distributed, called Jynneos, can provide protection from monkeypox after exposure, meaning it can benefit people who have already had contact with someone infected with monkeypox, as long as they get their first dose within 14 days.

The Jynneos vaccine is a two-dose regimen, with the doses spaced four weeks apart. Unlike the other vaccine that works for monkeypox — an older vaccine called ACAM200 — the Jynneos vaccine has relatively mild side effects, most commonly muscle pain, headache and fatigue.

Herlihy said Colorado placed an order Thursday for 1,000 more doses of Jynneos, showing how potential community spread of monkeypox is causing CDPHE to adjust its virus-fighting strategy.

Monkeypox 101

Monkeypox is endemic in parts of Africa but this year has spread to outbreaks in 50 countries, mostly in Europe and North America. Symptoms include fever, headache and muscle aches, as well as the telltale rash and sores, though the virus has been presenting itself in unexpected ways during the latest outbreaks. The version of the monkeypox virus that is spreading has a fatality rate of about 1%.

This 2003 electron microscope image made available by the Centers for Disease Control and Prevention shows mature, oval-shaped monkeypox virions, left, and spherical immature virions, right, obtained from a sample of human skin associated with the 2003 prairie dog outbreak. Monkeypox, a disease that rarely appears outside Africa, has been identified by European and American health authorities in recent days. (Cynthia S. Goldsmith, Russell Regner/CDC via AP)

Monkeypox is much less contagious than a virus like the one that causes COVID-19. It typically requires skin-to-skin contact or contact with something like bed sheets that have touched a rash sore. Health officials stress that the virus is not exactly sexually transmitted, but the intimate contact that occurs during sexual activity is sufficient to spread the virus.

Shifting strategies

Because it is less transmissible, the state’s strategy has been to contain cases as they pop up. Health officials had been administering the Jynneos vaccine to people known to have been exposed to monkeypox, hoping to fence the virus in and prevent it from breaking out into the wider community.

But now that the virus is spreading more widely and possibly undetected, the state has shifted to providing “prophylactic” vaccination to people who, for the moment, are most at risk.

Herlihy said she remains confident that Colorado can contain the monkeypox outbreak before it develops into something worse.

“I think we’re acting early, so I think that is a great thing,” she said, “and I think there is the potential for significant benefit.”

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 the...