The novel coronavirus has probably been spreading undetected for about six weeks in Washington state, where the first US death was reported this weekend.
A genetic analysis of the virus from a newly diagnosed patient in Snohomish County closely matched that of a specimen from the first known coronavirus patient in the United States, who traveled from China in January.
Trevor Bedford, a computational biologist at Fred Hutchinson Cancer Research Center in Seattle, announced his findings on Saturday night.
The evidence overwhelmingly suggests that these cases are linked through community transmission, Bedford wrote - and that this has been going on for weeks, with hundreds of infections likely.
"This strongly suggests that there has been cryptic transmission in Washington state for the past six weeks," he wrote. "I believe we're facing an already substantial outbreak in Washington State that was not detected until now due to narrow case definition requiring direct travel to China."
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The United States, Australia and Thailand each reported their first coronavirus-related deaths over the weekend, all in the span of about 12 hours. Covid-19 has killed patients on four continents, with the global death toll now reaching 3000.
The first US death, in Washington state, was a man in his 50s with underlying health conditions, officials said. The patient had no recent travel history or contact with people known to be infected, officials said.
Officials in the Seattle area announced two cases related to a possible outbreak at a long-term nursing home, Life Care Center in Kirkland. The elderly are considered to be especially vulnerable to infection.
One patient is a health-care worker in her 40s who was in satisfactory condition, according to state health officials. The other, a resident in her 70s, is in serious condition.
Earlier on Saturday, the Trump administration outlined new travel restrictions affecting Iran, Italy and South Korea in response to the outbreak, and President Donald Trump said he was considering further restrictions across the southern border.
Health and Human Services Secretary Alex Azar warned on Sunday that more coronavirus infections will surface in the United States. He said they are watching for possible drug shortages from disruptions to the pharmaceutical supply chain in China.
"We cannot make predictions as to how many cases we'll have, but we will have more, and we will have more community cases," he said on CBS' Face the Nation programme. "It's simply just a matter of math."