Washington had the United States’ first confirmed case of coronavirus, announced by the Centers for Disease Control and Prevention on Jan. 20. Based on an analysis of the virus’s genetic sequence, another case that surfaced in the state and was announced on Friday probably was descended from that first case.
The two people live in the same county, but are not known to have had contact with one another, and the second case occurred well after the first would no longer be expected to be contagious. So the genetic findings suggest that the virus has been spreading through other people in the community for close to six weeks, according to one of the scientists who compared the sequences, Trevor Bedford, an associate professor at the Fred Hutchinson Cancer Research Center and the University of Washington.
Dr. Bedford said it was possible that the two cases could be unrelated, and had been introduced separately into the United States. But he said that was unlikely, however, because in both cases the virus contained a genetic variation that appears to be rare — it was found in only two of the 59 samples whose sequences have been shared from China, where the virus originated.
A scientist who was not involved in the analysis said he agreed with the conclusion that the second case was connected to the original Washington case. “I think he’s right,” said Andrew Rambaut, professor of molecular evolution at the University of Edinburgh, referring to Dr. Bedford. “It’s extremely unlikely that two viruses coming from outside the U.S.A. independently would arrive in the same geographical area and be genetically related unless they were connected.”
State and local health officials have been hamstrung in their ability to test widely for the coronavirus. Until very recently, the C.D.C. had insisted that only its test could be used, and only on patients who met specific criteria — those who had traveled to China within 14 days of developing symptoms or had contact with a known coronavirus case.
If the virus has been spreading undetected in Washington since mid-January, that could mean that anywhere from 150 to 1,500 people may have it, with about 300 to 500 people the most likely range, said Dr. Mike Famulare, a principal research scientist at the Institute for Disease Modeling in Bellevue, Wa., who performed the analysis. These people “have either been infected and recovered, or currently are infected now,” he said.