COVID-19 was likely spreading in several US cities "far earlier" than Americans knew, The New York Times reported citing a new research by Northeastern University.

By the time New York City confirmed its first case of the coronavirus on March 1, "thousands of infections were already silently spreading through the city," said the report titled "Hidden Outbreaks Spread Through U.S. Cities Far Earlier Than Americans Knew, Estimates Say" posted Thursday on nytimes.com.

Hidden outbreaks were also "spreading almost completely undetected" in Boston, San Francisco, Chicago and Seattle, "long before testing showed that each city had a major problem," said the report, citing the results from a model of the spread of the disease by researchers at Northeastern University.

"Even in early February - while the world focused on China - the virus was not only likely to be spreading in multiple American cities, but also seeding blooms of infection elsewhere in the United States, the researchers found," the report said.

In the five major U.S. cities, as of March 1, there were only 23 confirmed cases of coronavirus, but according to the Northeastern model, there could have actually been about 28,000 infections in those cities by then, the report said.

Health authorities of Santa Clara County in the western U.S. state of California confirmed on Tuesday that two patients had died of COVID-19 at least three weeks before the first known U.S. death from the virus on Feb. 29 in Kirkland in Washington State.