Coronavirus Hit the U.S. Long Before We Knew

Retrospective testing in Ohio found that five women and a man who had developed Covid-like symptoms in early January had antibodies for the virus, qualifying as "probable" infections. They lived at opposite corners of the state, up to 200 miles apart. Though the findings aren't conclusive, modeling experts say they are consistent with the speed of the pandemic's later spread.

Screening began on air travelers, but only those coming from Wuhan, China, where the virus emerged, and only at airports in San Francisco, Los Angeles and at New York’s JFK. A handful of sick people were identified, but scientists say more were missed. By the end of January, more than half a million people flew from China to the U.S.

While the country was still looking outward, researchers now believe the earliest infections in many states came mostly from travel within the U.S.

By now the virus was likely spreading to 10 new people every day in California, according to estimates from researchers at Northeastern University’s Network Science Institute.

With the virus creeping across the U.S., the country barred arrivals by non-U.S. residents from China.

The disease was taking root in New York. Later genetic analysis shows that by this date at least seven distinct strains could have arrived in New York City, mostly from Europe and the U.S.

A 57-year-old woman died in Santa Clara County, Calif. Later tests found she had the coronavirus, making her the first known U.S. Covid-19 death. It’s still not known how she caught the virus.

U.S. testing had largely stopped earlier this month due to problems with test kits. Odds were good the virus had taken hold in as many as 15 states, Northeastern researchers found. In Colorado, the spread possibly began among skiers arriving for peak season, according to David Bortz, a professor of applied mathematics at the University of Colorado and member of the state’s Covid-19 modeling team.

By now it was more likely than not that local transmission of the virus was underway in 24 states.

A swab collected from a door handle of a public building in Gainesville, Fla., for a routine flu study was later found to carry the virus. Genetic analysis showed the strain was very closely related to samples detected in the Puget Sound area of Washington this same week.

How did the disease spread so rapidly? Large gatherings, like the Biogen conference in Boston on Feb. 26-27, played a role. A strain likely imported from France spread among attendees. Genetic reconstruction and case histories show they carried it to North Carolina, Tennessee and Texas. Sequencing shows the strain also became established in Massachusetts and Virginia.

As February came to an end, there was a better-than-even chance of local transmission of the virus in 37 states, models from Northeastern researchers show.

Why was the country in the dark? A lack of testing played a part. Researchers think we were catching a much lower percentage of symptomatic Covid-19 patients in mid-February than at the beginning of the month.

Cruises were another vector. In early March, attention focused on the Grand Princess, a ship blocked from docking in San Francisco. But beginning Feb. 11, more than 100 people who later tested positive for Covid-19 set sail on nine different cruises on the Nile River. They scattered to 18 states and the District of Columbia, the U.S. Centers for Disease Control and Prevention said.

The agency didn’t ban cruises in U.S. waters until March 14, two days before the U.S. closed the door to the arrival of most air travelers from Europe. By this point, the virus was firmly embedded across the country.