EVERETT — The Snohomish County Health Department declared a measles outbreak Thursday after confirming three new measles cases this week.

The three infected children were exposed to a contagious family visiting from South Carolina over the holidays and tested positive Wednesday for the virus that causes measles. Last week, the health department announced that three members of the family had tested positive and visited multiple locations in Everett, Marysville and Mukilteo.

The children who tested positive Wednesday were unvaccinated and experienced fever, cough and rashes consistent with measles, the health department said in a press release. There were two confirmed cases at Mukilteo School District — one at Serene Lake Elementary School and another at Pathfinder Kindergarten Center.

Serene Lake Elementary School will be closed Friday to provide the district time to work with the health department on next steps, Mukilteo School District Superintendent Alison Brynelson said in a message to families Thursday. The district will share updated information by the end of the day Friday.

Unvaccinated children should quarantine at home and avoid all public places until Jan. 30, Brynelson said. The district is asking families to monitor their children for symptoms through Jan. 30. For further instructions and support during quarantine, families can call the health department’s communicable disease program at 425-339-3503.

“We’re expecting more measles cases as a result of the previous exposures as well as these new outbreak cases,” said Snohomish County Health Officer Dr. James Lewis in the release. “We are hoping to contain it, but unfortunately there have been a number of exposure sites already and there are likely more cases in the community that we do not know about yet.”

A measles outbreak is when three or more people in the same community get measles that has spread from one person to another, the release said. Lewis conferred with the state health department before declaring the outbreak.

The measles virus can remain in the air for up to two hours after someone with measles leaves the area. Anyone who was at the following locations at the times listed could have been exposed to measles:

• 8:30 a.m. to 5:15 p.m. Jan. 9 at Pathfinder Kindergarten Center, 11401 Beverly Park Road, Everett

• 8:30 a.m. to 5:15 p.m. Jan. 9 at Serene Lake Elementary School, 4709 Picnic Point Road, Edmonds

• 1:10-3:10 p.m. Jan. 13 at Swedish Mill Creek Campus medical facility, 13020 Meridian Ave. S., Everett

Most people in Snohomish County have immunity to measles through the measles, mumps and rubella vaccine, the release said. Health officials are urging residents to check if they’ve received the measles vaccine.

The vaccine is safe and effective, according to health officials. Two doses of the vaccine are about 97% effective at preventing measles. The protection lasts a lifetime.

In South Carolina, 434 measles cases have been reported since October, making it the largest active measles outbreak in the United States, according to the South Carolina Department of Public Health.

“We’re concerned about the health and safety of our Snohomish County children,” Health Department Director Kim Van Pelt said. “Measles is a serious disease that often brings unpleasant symptoms, and in rare cases swelling of the brain and even death.”

Those who think they may have been exposed to measles and haven’t been vaccinated should call a health care provider as soon as possible if they develop an illness with a fever or unexplained rash, the release said. Limit contact with others and call before going to a clinic or hospital.

Measles symptoms include a fever, rash, cough, runny nose, and red, watery eyes. It can lead to ear infections, diarrhea and pneumonia. Rarely, it can lead to brain inflammation or death.

Measles is highly contagious. If one person has it, up to 9 out of 10 people will become infected if they aren’t vaccinated, according to health officials. Measles is contagious from about four to five days before a rash appears to four days after the rash appears. The incubation period for measles is 10 to 14 days from exposure to onset of symptoms.

Those at the highest risk for complications from measles include infants and children under five years old, adults over 20 years old, pregnant people and people with weakened immune systems from medications or underlying disease, according to the health department.

In 2000, measles was declared eliminated from the United States after the measles vaccine was implemented. But cases have increased in recent years, data from the Centers for Disease Control and Prevention shows. In 2025, 2,242 cases were reported to the CDC, the highest level since 1992.

More information about measles is available at www.snohd.org/460/Measles.

Correction: A previous version of this story listed an incorrect address for Serene Lake Elementary School. The correct address is 4709 Picnic Point Road, Edmonds.

Jenna Peterson: 425-339-3486; jenna.peterson@heraldnet.com; X: @jennarpetersonn.