People who took the Regional Transportation District’s Flatiron Flyer between Boulder and Denver last week should watch for measles symptoms after a contagious Boulder County resident rode that route six times.

The unidentified resident returned from a recent trip to Europe with measles, becoming the state’s 15th case this year, according to the Colorado Department of Public Health and Environment.

The spike in measles cases in Colorado, the most the state has recorded in decades, comes after a deadly outbreak in Texas this year, and as other hot spots have emerged around the world for the highly contagious disease.

The infected Boulder County resident was an adult who was fully vaccinated, health officials said. Two shots of the vaccine are about 97% effective in preventing infection.State health officials say people should monitor themselves for symptoms for 21 days — and consider avoiding public gatherings — if they visited the following exposure locations in Boulder and Denver:

• RTD Flatiron Flyer between Denver and Boulder, both ways, from 6:45 to 9:35 a.m. and 4:05 to 7:15 p.m. June 3, June 5 and June 6.

• Market Building, 1400 16th St., Denver, from 8 a.m. to 6:15 p.m. June 3-6.

• Little Finch Coffee Shop, 1490 16th St., Denver, from noon to 2:15 p.m. June 4 and June 5.

• Whole Foods, 1701 Wewatta St., Denver, from noon to 2:30 p.m. June 4.

• Rosetta Hall, 1109 Walnut St., Boulder, from 7:30 to 10:45 p.m. June 6.

Early symptoms include a cough, runny nose, red eyes and a fever, with a red rash starting about four days after someone becomes contagious. People can begin to feel sick one to three weeks after exposure.

People who feel ill after a possible exposure should call their health care provider or an urgent care center before seeking help, so the provider can reduce the risk the virus will spread to other patients.

Colorado has recorded at least eight measles cases tied to Denver International Airport and a Turkish Airlines flight that landed there with a contagious passenger on May 13. Other cases have been confirmed in Denver, Pueblo and Archuleta County.

In a typical year, Colorado has two or fewer measles cases. The last time the state recorded more than 10 in a year was in the 1990s.