The B.C. Centre for Disease Control is warning British Columbians to take precautions if they’re planning to travel to Washington state, where a state of emergency has been declared due to a measles outbreak in Clark County.

No cases of measles have been reported in B.C. in relation to the outbreak in Washington state, where at least 31 people have been infected.

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The number of confirmed measles cases near Portland grew to 31 on Saturday, an outbreak boosted by lower-than-normal vaccination rates in what’s been identified as an anti-vaccination U.S. “hot spot.”

Of the 31 cases in Clark County, 27 cases involve people were not immunized. One person has been hospitalized.

Public health officials in southwest Washington, just across the Columbia River from Portland, Oregon, said people may have been exposed to the dangerous disease at more than three dozen locations , including Portland International Airport, a Portland Trail Blazers game, an Amazon Locker location and stores such as Costco and Ikea.

Canadians travelling to the affected communities are at risk of exposure to measles, according to the BCCDC. People with measles can infect others before the onset of symptoms like fever and rash, the centre said. “While it is expected that most travellers will be immune to measles, some individuals will be susceptible, including infants less than one year old or people who have never been immunized against measles,” the centre said in a news release.

Measles is highly infectious and spreads through the air by coughing and sneezing.

The best protection against measles is vaccination, the centre said.

British Columbians can update their immunization status at any time, especially before travelling. The measles vaccine is available as a combined measles, mumps and rubella vaccine, and is available from your local health unit, family doctor, and many pharmacists. To find a public health unit anywhere in the province, see the site finder on ImmunizeBC.ca.

In B.C., all school-age children, university students and those traveling out of the country and anyone born after Jan. 1, 1970 should have received two doses of measles-containing vaccine, as should health-care workers born on or after Jan. 1, 1957.

B.C. typically experiences a few cases of measles each year, typically a result of under-vaccinated travellers returning from parts of the world where measles is still common, the BCCDC said. In 2018, six cases of measles were reported by B.C. residents, two cases acquired during travel to India and Philippines. In 2019, a single case of measles was reported by an adult traveller returning from the Philippines.

Washington Gov. Jay Inslee, a Democrat, declared a statewide public health emergency for his state on Friday and authorities in neighbouring Oregon and Idaho have issued warnings to residents.

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

Clark County, which includes the Portland bedroom community of Vancouver, Washington, has a measles vaccination rate of 78 per cent, well below the 92 to 94 per cent rate required for so-called “herd immunity,” said Marissa Armstrong, the department’s spokeswoman.

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 two-year-olds in Multnomah County, home to Portland, was 87 per cent in 2017, according to state data. The measles vaccine consists of two shots, one given by age two and the second usually between ages four and six.

Two doses of the vaccine in childhood are 97 per cent effective and provide lifetime immunity. One dose is about 93 per cent effective.

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 per cent.

In B.C., an analysis of the most recent years of data available show 87.3 per cent of children turning two had received one dose of measles vaccine, and 88.4 per cent of children turning seven had received two doses, according to the B.C. Centre for Disease Control. A level of 95 per cent is recommended in order to prevent the diseased from spreading, the centre said.

People who have the measles can spread the disease for several weeks before they show symptoms.

The virus can remain in the air for up to two hours in an isolated space. Ninety per cent 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.”

Authorities have been successful in identifying several people who had been exposed but were not sick yet. Those people stayed home and later got ill, Armstrong said.

“We’re taking that as a win. That’s how we’re going to be able to slow this down is keeping these people out of a public setting,” she said. “People are helping us and staying home.”

Officials still are not sure where the outbreak began.

The first known patient sought medical care on Dec. 31, but it is unknown if other people may have gotten sick before that and did not seek treatment.

Those who may have been exposed should watch for early symptoms of fever and malaise and then a rash starting on the head and moving down the body. Serious complications such pneumonia and brain infections can arise from the disease in some cases.

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

With files from Gillian Flaccus/The Associated Press