(Reuters) - Seattle-area health officials confronting the nation's first community coronavirus outbreak are planning new containment measures, ranging from possible school closings to temporary quarantine housing for mildly ill homeless patients.

The shift in strategy, with an emphasis on enlisting the public at large to take a more active role in curtailing the spread of the virus, came as health authorities announced on Monday that 18 Washington state residents had tested positive, including six who died.

"We're pivoting to a more community-based approach, very similar to what we use for influenza epidemics, where we give people, and schools and businesses good advice on how they can reduce their risks," Dr Jeff Duchin, health officer for Seattle and King County's public health agency, told reporters.

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The Seattle-area deaths marked the first fatalities documented in the United States from a respiratory disease that has killed more than 3,000 people worldwide - the bulk of them in China - since it emerged there in December.

All of Washington's cases are clustered in two counties in the greater Seattle area, making it the largest concentration detected to date by the U.S. public health system. Eight of those cases, and four of the deaths, were linked to a nursing care facility in the Seattle suburb of Kirkland, officials said.

With the virus now believed to be present in at least four separate communities in the Pacific Northwest - two in northern California, one in Oregon and the Washington outbreak - authorities are having to go well beyond the quarantine of infected travelers and tracing of close contacts that has defined the response.

"We're going to be shifting our approach from counting every case to focusing on outbreaks, perhaps cases that occur in hospitals, and won't be able to do the kind of individual case follow-up and case management that we're doing early on during this so-called containment phase of the epidemic," Duchin said.

EXPANDED TESTING

Health officials predict the number of confirmed cases is certain to rise as testing capacity expands, and the true scale of the outbreak comes into sharper relief. Washington state plans to increase testing from about 200 samples a day now to an expected volume of more than 1,000 a day.

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Doctors have found that patients most severely stricken by the virus are those with immune systems weakened from advanced age or underlying chronic health conditions, Duchin said.

The vast majority of infected individuals are believed to suffer relatively minor flu-like illness. Many such cases have likely gone undetected because initial testing efforts, constrained by a limited supply of resources, were reserved for the few who were severely ill, he said.

So the patients who end up hospitalized or dead probably represent "the tip of the iceberg," Duchin said.

King County Executive Dow Constantine said he has ordered the purchase of a motel and the placement of modular housing units on public property throughout the county to provide temporary, quarters for homeless people who become ill - but not so sick that they require hospitalization.

"We want to make sure that hospital capacity isn't being taken for people who need only to be in isolation and in recovery," he said. "We need the hospital capacity for people who need to be in treatment now."

Seattle has one of the largest concentrations of unhoused people of any major metropolitan area in the country.

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Beyond such contingencies, public health officials are starting to contemplate possible "community mitigation" measures, such as school closures or cancellation of public events.

"We want to make sure that what we do is reasonable, that it's something that's acceptable to the public ... that the timing and duration are right, and all of this in the face of tremendous uncertainty," Duchin said.

As of Monday, there have been more than 89,000 cases of the virus globally, the majority in China, according to a Reuters tally. Outside China, it has spread to 66 countries, with more than 8,800 cases and 130 deaths. Globally, the illness has killed more than 3,000 people.

The U.S. Centers for Disease Control and Prevention lists more than 90 cases across the United States, a large bulk of them patients who were repatriated from the Diamond Princess cruise liner that had been quarantined in Japan.

(Reporting by Steve Gorman; Editing by Bill Tarrant, Robert Birsel)