SACRAMENTO, CALIFORNIA (REUTERS) - The novel coronavirus circulated in January in California, weeks earlier than thought, and early deaths were likely mistaken for the flu, a county health official said on Wednesday (April 22).

A 57-year-old woman had died of Covid-19 on Feb 6, far earlier than any other reported cases in the United States, said Sara Cody, the health officer in Santa Clara County, California.

It was previously thought that the first US death from Covid-19, the respiratory disease caused by the virus, was in Washington state on Feb 29.

News of the deaths in California could improve public health officials' understanding of how the outbreak took hold in the United States.

The California woman's death and two other early cases - a 69-year-old man who died Feb 17 and a 70-year-old man who died March 6 - were confirmed to have been Covid-19 by the US Centres for Disease Control and Prevention after it tested tissue samples.

The county had previously identified its first case of community transmission - infectious spread among people who had not been to China or other early hot spots - on Feb 28, Cody said.

But none of the three patients who died had travelled.

"What these deaths tell us is that we had community transmission probably to a significant degree, far earlier than we had known, and that indicates that the virus was probably introduced and circulating in our community far earlier than we had known," Cody said.

Because the region was undergoing a bad flu season at the time, many cases may have been misclassified as influenza, she said.

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The cases were likely "iceberg tips", Cody said, indicating that many more people were also infected.

The three cases were discovered because the county medical examiner's office was not satisfied that it had found the correct cause of death, Cody said.

Because coronavirus tests were not available, they saved tissue samples, which they sent to the CDC.

The testing parameters at the time by the CDC restricted testing to individuals with a known travel history and who sought medical care for specific symptoms.

US coronavirus deaths topped 46,000 on Wednesday, doubling in a little over a week and rising on Tuesday by a near-record amount in a single day, according to a Reuters tally.

The United States has by far the world's largest number of confirmed coronavirus cases at more than 820,000.