The number of measles cases confirmed near the city of Portland increased to 31 on Friday, and there are nine other people suspected of having the disease.
The outbreak of measles has been enhanced by a lower vaccination rate than usual in what has been identified as a hotbed of opponents of vaccination in the country.
Health officials in southwestern Washington state, across the Columbia River, said people could have been exposed to the dangerous disease in more than thirty places, including the Portland International Airport, an NBA-run the Trail Blazers, an Amazon Locker location and in stores like Ikea and Costco.
Twenty-six of the confirmed patients had not been vaccinated against measles, and the vaccination status of four others who became infected is not known. One minor was hospitalized.
A case was confirmed in King County, where the city of Seattle is located, and one was confirmed Friday night in Multnomah County, where Portland is located.
Most cases involve children under the age of 10, the Clark County Department of Public Health said in a statement. One of the infected is an adult and the rest are teenagers. The Oregon authorities did not provide the age of the infected adult.
Washington Governor Jay Inslee declared a statewide health emergency on Friday and authorities in neighboring states of Idaho and Oregon also issued alerts to residents.
The number of cases “creates a serious public health risk that could spread rapidly to other counties,” Inslee said.
The measles vaccine has been part of the childhood vaccination scheme for decades, and the disease was declared eradicated from the United States in 2000.
But measles is still a big problem in other parts of the world. Travelers infected abroad can bring the virus to the country and spread it, causing periodic outbreaks.
Last year there were 17 outbreaks and 350 cases of measles in the United States.
Before the vaccine, between 400 and 500 people died of measles each year in the country, 50,000 people were hospitalized and 4,000 developed brain inflammation that could cause deafness, said Dr. Alan Melnick, director of public health for Clark County on Friday.
Clark County, which includes the residential area of Vancouver, in Washington, has a measles vaccination rate of 78%, well below the 92% or 94% that is required for so-called “collective immunity”, said Marissa Armstrong, spokeswoman for the Department of Health.
Collective immunity is achieved when individuals who are vaccinated are protected from infection because almost everyone around them has been vaccinated, and they become immune to the disease.
The measles virus, which spreads 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 sick, health officials said.