The first coronavirus victim in the US died of a "ruptured heart" as her body fought the spread of Covid-19 throughout her heart, lungs, trachea and intestines, according to an autopsy report.

California woman Patricia Dowd, 57, died on 6 February of an apparent heart attack while suffering from flu-like symptoms. Officials later confirmed she had coronavirus, making her the country's first known Covid-19 fatality and suggesting the virus was circulating weeks earlier than previously thought.

"The immune system was attacking the virus and in attacking the virus it damaged the heart and then the heart basically burst," said Dr Judy Melinek, a forensic pathologist who reviewed the autopsy report for The Mercury News.

Melinek said Ms Dowd's body struggled so hard against the virus but her heart muscle was infected, causing a valve to rupture.

Santa Clara County medical examiner Dr Patricia Cabello wrote in the official autopsy that the Sars-CoV-2 infection was detected in the heart, trachea, lung and intestines.

"Cause of death: Acute hemopericardium due to rupture of left ventricle due to transmural myocardial ischemia (infarction) with minor component of myocarditis due to Covid-19 infection," she wrote in the autopsy, first reported by The San Francisco Chronicle.

Ms Dowd, a manager for a semiconductor company, appeared to have recovered from her flu-like symptoms before dying suddenly of what was first believed to be a heart attack.

When health officials confirmed the death of Ms Dowd on 6 February was due to Covid-19, it moved-forward the timeline of when the virus was widely circulating in the country -- as early as mid- to late-January based on average times between infection and death.

The first case of community transmission was announced on 28 February, while the first death was previously considered to be in Seattle, Washington in late February.

The Bay Area didn't begin issuing stay at home orders until 17 March, nearly six weeks after Ms Dowd's death.

"If we had understood then that people were already dying... we probably would have acted earlier than we did," said Santa Clara County Health Officer Dr Sara Cody at a press briefing last week.