Vaccinated adult from Colorado infected with measles, the state’s first “breakthrough” case this year – The Colorado Sun
2025-04-25T23:21:58Z
A vaccinated Denver resident who had recently traveled to Mexico was confirmed as the state’s fifth measles case of 2025
Colorado on Friday reported its fifth incidence of measles this year, and, for the first time in the spate of cases, the infection was in a person who had been vaccinated against the disease.
The person, an adult living in Denver, had recently traveled to the Mexican state of Chihuahua, which is seeing an outbreak of measles. Two other cases identified in Colorado — one in a baby in Denver and another in a Pueblo resident — were also connected to travel to Chihuahua.
The Colorado Department of Public Health and Environment identified one location where people may have been exposed to the virus as a result of this case.
Anyone who was at the Kaiser Permanente urgent care clinic at 8383 W. Alameda Ave. in Lakewood between 10:30 a.m. and 1 p.m. on Saturday, April 19, should monitor themselves for symptoms for 21 days and consider avoiding public gatherings.
If you think you may be sick with measles, call ahead to your doctor’s office or emergency clinic before going in to allow the staff to make sure you can be seen without risk of exposing others.
About “breakthrough” measles infections
The new case is unusual because it is in someone who has verified proof of MMR vaccination. But that doesn’t mean such a so-called breakthrough infection is unexpected, said Dr. Ned Calonge, CDPHE’s chief medical officer.
Two doses of MMR vaccine are considered 97% effective at stopping an infection — one dose is considered 93% effective. That means on average three out of every 100 vaccinated people who are exposed to the virus will get sick.
When that happens, the illness is likely to be more mild, and that is exactly what occurred in the latest Colorado case, Calonge said. He said the infected person is reported to be feeling well.
Vaccinated people who develop an infection are also less infectious toward others, Calonge said.
Calonge emphasized that this breakthrough infection does not mean the virus has mutated dangerously or become more adept at evading the immune system. Rather, he said, in rare cases people will not respond to the MMR vaccine or their immunity will wane over time.
Faster notification
While state officials are optimistic about the health system’s readiness to respond to measles infections, some of the earlier cases saw long delays in reporting suspected infections. That caused CDPHE to send out reminders to clinics and labs that possible measles cases need to be reported promptly to the state.
A measles virus particle, artificially colored in red, is seen in an image taken through an electron microscope. (National Institute of Allergy and Infectious Diseases, via Flickr)
In the latest case, it took nearly a week from the time the person visited the clinic to when the public was told about the confirmed case of measles. But Calonge said the clinic did inform CDPHE right away.
The delay in publicly reporting the infection was because health officials looked at the vaccination status of the person and weren’t certain the test result to confirm a measles infection would come back positive.
“The decision was, based on the vaccination history, that the likelihood was low,” Calonge said.
Regardless, Calonge said anyone who has been tested for measles should isolate themselves until the test result comes back.
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