Health insurance premiums for millions of Americans could skyrocket by more than 40 per cent in 2021 without significant federal intervention as the US health system surges with coronavirus patients.

Projected costs for 170m Americans with health insurance on the commercial market — including individual and employer-based group insurance plans — range from $34bn to $251bn within the first year of the pandemic, according to Covered California, the state's health insurance marketplace through the Affordable Care Act.

The group's analysis offers one of the first nationwide health insurance cost scenarios in the wake of the pandemic that has sickened thousands of people across the US.

Peter Lee, director of Covered California, says that "absent federal action, consumers, employers and our entire health care system may be facing unforeseen costs" that could exceed $250bn.

He says that "consumers will feel these costs through higher out-of-pocket expenses and premiums" as well as employers dropping coverage entirely or shifting the cost burdens to employees, which could force many Americans to dump their plans altogether if they become too costly.

"These are not 'insurer' costs — these are costs directly borne by individual Americans in the form of cost-sharing and premiums", Mr Lee said. "These are costs to small and large businesses that are struggling. These are costs to individuals who may avoid needed care."

While hospitals have seen a rise in severely ill patients, care costs are also spread out among patients experiencing more minor symptoms, or patients who may develop pneumonia and require hospital stays and the use of expensive equipment.

Estimates that patients who are hospitalised for up to 12 days because of coronavirus could face bills of $72,000, significantly higher than other projected cost averages around the US, according to Covered California.

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The group examined three impact scenarios for the coronavirus a low-impact crisis would hospitalise 400,000 people, a medium impact would hospitalise 1.2m people, and a "high" impact would hospitalise 3m people.

Insurers will begin setting 2021 rates in May and finalising plans by July, leaving a short window of time for congressional action to prevent massive premium hikes and avoid overwhelming cost surges, according to Covered California's chief actuary John Bertko.

"While there is a lot of uncertainty with anything related to Covid-19, one thing we can be certain of is that the impact will be significant, and now is the time to take action", Mr Bertko said.

The estimates arrive on the 10th anniversary of the ACA, landmark federal health legislation that expanded private insurance options to millions of Americans through a government marketplace, and extended Medicaid eligibility states with a "gap" in coverage for lower-income people.