Two people who died in California in early and mid-February were confirmed as coronavirus cases — weeks before the first reported Covid-19 fatality in the United States, it emerged on Wednesday.

Autopsies on the two individuals who died at home on February 6 and February 17 showed they had succumbed to Covid-19, the Santa Clara County coroner said, after receiving confirmation from the Centres for Disease Control and Prevention (CDC).

Another person who passed away in Santa Clara County on March 6 also died of the disease caused by the coronavirus, the medical examiner-coroner said in a statement.

"These three individuals died at home during a time when very limited testing was available only through the CDC," the coroner said.

"Testing criteria set by the CDC at the time restricted testing to only individuals with a known travel history and who sought medical care for specific symptoms."

The coroner said they expected to identify more coronavirus-related fatalities in Santa Clara. "As the medical examiner-coroner continues to carefully investigate deaths throughout the county, we anticipate additional deaths from Covid-19 will be identified."

The earliest death attributed to the coronavirus was previously thought to be on February 26 in Washington state.

Health experts say a shortage of test kits means the United States may be underestimating the extent of the virus outbreak.

A study this week indicated that the novel coronavirus was likely far more widespread than official figures suggest.

Read: US has three times more patients than any other country

Blood samples taken from 3,300 volunteers in Santa Clara County showed the true number of Covid-19 cases was at least 50 times higher than the number of confirmed infections in the county, according to the study by Stanford researchers.

Santa Clara was among the first areas in the US to tell residents to stay at home to contain the spread of the virus, ordering a shutdown from March 17.

The United States has recorded more than 800,000 confirmed cases of Covid-19 since the start of the pandemic, according to a tally from Johns Hopkins University, with over 45,000 deaths — the most reported of any country.