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, a hidden explosion of a disease that many still viewed as a remote threat as the city awaited the first signs of spring.

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, according to a model of the spread of the disease by researchers at Northeastern University who shared their results with The New York Times.

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.