Measles cases in Orange County this year have surged to 20 and public health officials said Monday they expect more people to catch the highly contagious disease.

County Health Care Agency spokesman Nicole Stanfield said residents should make sure they have been vaccinated for measles, mumps and rubella. The vaccine is 99 percent effective after two doses.

The agency previously had been issuing notices of potential places of exposure, but said the list is now too long.

“We know it’s in the community,” Stanfield said. “The more cases we have, the more people can potentially be exposed.”

Stanfield said for the 10 most recent cases, seven adults and two children were unvaccinated. The vaccination history of the tenth adult is unknown. The recent cases had come into contact with earlier measles patients, she said.

“The important thing is people need to check to make sure they’re vaccinated,” Stanfield said.

As of Friday, statewide there had been 36 measles cases reported, although that doesn’t include the most recent numbers from Orange County. Last year at this time in California, there had been four reported cases of measles, the Department of Public Health said.

In an interview last week, Dr. Matt Zahn, the county’s medical director for epidemiology, said the outbreak has increased demand for the MMR vaccine at doctors’ offices and the county’s vaccination clinic in Santa Ana.

In a typical year, Orange County sees no cases or one case of measles, an illness that was declared eliminated in the U.S. in 2000.

Measles symptoms can develop between seven and 21 days after exposure, starting with a fever, cough and runny nose. Two to four days later, a red, blotchy rash develops, usually starting around the ears and hairline and then moving to the face, torso and limbs. Measles is contagious for four days before the rash develops and until four days after.

The Centers for Disease Control and Prevention recommends that children are vaccinated at 12 to 15 months and again at age 4 to 6 before starting school.

Contact the writer: 714-796-3686 or cperkes@ocregister.com