A Seattle expert is estimating about 20,000 people in the U.S. are now infected with the new coronavirus, nearly 10 times more than the roughly 2,300 confirmed cases.

Trevor Bedford, a computational biologist at the Fred Hutchinson Cancer Research Center, posted his estimates in a series of 13 tweets Friday night. “I could easily be off 2-fold in either direction, but my best guess is that we’re currently in the 10,000 to 40,000 range nationally,” he wrote.

The estimate is based on evidence that about 60 cases were imported to the U.S. from the Wuhan area of China from Jan. 15 to Feb. 15, only a quarter of which were detected. Of those other 45 cases, which were probably mild, many might have fizzled out, leading to no new infections. Bedford makes a rough guess that about 20 of those “sparks” caught hold and began to spread to other people. Those transmission chains have been percolating quietly for four to eight weeks and are now starting to result in exponential growth of new infections, he writes.

With the number of infections doubling every five to six days, each of those 20 initial sparks has probably grown to an average of 1,000 cases by now, Bedford estimates – accounting for the rough estimate of 20,000 infections nationwide.

“Again, there are a bunch of assumptions here, and the main point is not the (approximately) 20k number, but instead that what we’re seeing now are not new introductions, but small outbreaks that have been growing locally over the past (approximately) 8 weeks and now getting big enough to be noticed.”

On March 12, after President Donald Trump banned travel to the U.S. from many European countries, Bedford posted another series of tweets, suggesting the ban would have little impact. Most of the new infections in the U.S. are being spread by people who are already here, he calculates. “I believe the focus needs to be on testing and case finding in the US to slow transmission here,” he wrote. “Introductions through air travel are likely to have a more minor impact at this point.”

On Friday, the Fred Hutch posted a Q&A with Bedford, one of the first times he’s spoken at length about the coronavirus work that has landed him in the international spotlight.

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