There are now 55 confirmed cases of measles in the Northwest, including 50 clustered in southwest Washington, one in Seattle, and four in Multnomah County, Oregon, according to health officials. Eleven suspected cases were reported Wednesday.

The four confirmed cases in Multnomah County are linked to the outbreak in Washington. Health officials in Oregon said Wednesday the four cases pose no risk to the public.  

Health officials also identified one confirmed case of measles in a person who received one dose of the MMR vaccine. Of the confirmed cases, most patients were under 10 and at least 43 patients were not immunized. 

“The measles vaccine isn’t perfect, but one dose is 93 percent effective at preventing illness,” said Clark County Health Officer Dr. Alan Melnick. “The recommended two doses of the measles vaccine provide even greater protection – 97 percent.”

Health officials said people infected with the extremely contagious illness traveled to Hawaii and central Oregon after being exposed.

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The revelation prompted public health officials in Oregon's Deschutes County and in Hawaii to issue alerts, although no cases were confirmed in either location.

"It raises concerns that this can go on for a long time, become geographically larger than it is and more cases over weeks and months," said Dr. Alan Melnick, public health director for Clark County, in southern Washington, which is at the epicenter of the outbreak and has a lower-than-normal vaccination rate.

The two children who traveled to Hawaii were not contagious when they flew, Melnick said, and they were quarantined in the islands once they arrived. They have since returned home.

Officials haven't yet determined how the measles outbreak started. The first patient sought medical care on Dec. 31, but other sick people may not have gone to a doctor or hospital, he said.

Clark County, where the first case was documented, has a 78 percent vaccination rate — far below the 95 percent required for "herd immunity" for such a contagious virus.

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Herd immunity, or community immunity, is when enough of the population is vaccinated to protect those who haven't been vaccinated for medical reasons or because they are too young.

"If you have a large unvaccinated population and you add measles to the mix, one measles case will infect 90 percent of contacts, and the early symptoms are not distinguishable from other respiratory illnesses — and you're contagious at that point," Melnick said.

"So it's like taking a lighted match and throwing it into a bucket of gasoline, basically," he said.

Washington Gov. Jay Inslee declared a state of emergency because of the outbreak, and the federal Centers for Disease Control and Prevention has been consulting with local and state officials.

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The vaccine has been part of routine childhood shots for decades, and measles was declared eliminated in the U.S. in 2000. But it is still a big problem in other parts of the world, and travelers infected abroad can bring the virus back and spread it, causing periodic outbreaks.

Last year, there were 17 outbreaks and about 350 cases of measles in the United States.

Before mass vaccination, 400 to 500 people in the U.S. died of the measles every year, 50,000 people were hospitalized and 4,000 people developed brain swelling that can cause deafness, Melnick said. One to three cases out of every 1,000 are fatal, he said.

People who think they may have the measles should call their health care provider before showing up so the facility can take steps to limit other people's exposure.

Early symptoms include a fever, runny nose and malaise, followed by a red rash that starts around the head and moves down the body.

Patients are contagious four days before and four days after getting the rash.

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Inslee said the number of cases "creates an extreme public health risk that may quickly spread to other counties."

Herd immunity happens when unvaccinated individuals are protected from infection because almost everyone around them has been vaccinated and is immune to a disease.

The measles vaccination rate for 2-year-olds in Multnomah County, home to Portland, was 87 percent in 2017, according to state data. The measles vaccine consists of two shots, one given by age 2 and the second usually between ages 4 and 6.

Data on Portland's vaccination rate for both shots wasn't immediately available.

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Both Washington and Oregon allow vaccine exemptions for personal and philosophical reasons. Armstrong said the vaccine exemption rate in Clark County for non-medical reasons was high, at 7.5 percent.

People who have the measles can be contagious for four days before showing symptoms.

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The virus, spread by coughing or sneezing, can remain in the air for up to two hours in an isolated space. Ninety percent of people exposed to measles who have not been vaccinated will get it, public health officials said.

Every time an unvaccinated person who's been exposed to measles goes out in public, "it starts that clock over again," Armstrong said. "That's the fear."

People who think they may have the measles should contact their health care provider before visiting to avoid exposing others.