Through genetic analysis of virus samples, infectious disease detectives said Friday that the coronavirus cases that appeared on the Grand Princess cruise ship are very likely traceable back to the original Seattle case that spurred the hot spot to our north — and it's therefore likely that more Bay Area cases can be traced back to the Seattle area as well.

ICYMI, genetic sequencing has already occurred on dozens of cases in Seattle that have been traced back to the 35-year-old man being called Patient Zero — though he's surely not the only Patient Zero in the American chapter of this pandemic. As Bloomberg reported, the man arrived back in the U.S. from Wuhan, China on January 15, and he went to an urgent-care facility on January 19 after feeling ill and seeing a CDC alert about the coronavirus. He was fully recovered by January 21, but during the six days in which he was apparently that region's first case of COVID-19, the highly contagious pathogen infected numerous other people, and cascaded into the outbreak they have now — with 37 deaths to date in Washington State and at least 570 cases.

The outbreak in California is clearly behind the one in Washington, and as the Mercury News reports, scientists have looked at the small mutations in virus samples from patients that were on the Grand Princess cruise ship and similarly linked them back — in all likelihood — to that first Seattle case.

The Placer County man who arrived back in San Francisco on the ship on February 21 and later died first boarded the ship on February 11 — though it can't be proven whether he brought the virus onto the ship or someone else did. Some crew members also became ill on that same leg of the cruise.

Some of the Bay Area outbreak, the scientists suggest, could have been fueled by the other Grand Princess passengers who also arrived home on February 21 and unwittingly carried the virus with them.

"The simplest explanation is that the cases in the cruise ship were introduced from the cases in Washington state," says Dr. Charles Chiu, a professor of laboratory medicine at UCSF who is leading the genetic sequencing project, speaking to the Mercury News. Chiu explains that the genetic sequences of the virus samples from Seattle and the Bay Area are too closely matched to suggest any other explanation.

There were only five cases of the virus in the U.S. as of January 28, and on January 31 we recorded the first Bay Area case, a man in Santa Clara County. It wouldn't be until February 26 that we had the first documented case of community spread of the virus, and it was a woman in Solano County — near where hundred of evacuees had landed in quarantine from a different cruise ship, the Diamond Princess.

On March 5, only a week ago, we recorded the first two cases of COVID-19 in San Francisco proper, both the result of community spread. That number now stands at 23, with some cases traced to other known cases. The total number of Bay Area cases now stands at 165.

In related news, President Trump announced Friday that all four major cruise companies are suspending trips to and from the U.S. for the next 30 days.

Photo: Princess Cruises