Coronavirus cases are surging in the US, passing three million infections. And a key public health measure can no longer be used.

Coronavirus is spreading so rapidly in the southern United States that contact tracing is no longer possible, an expert says.

Dr Peter Hotez from Baylor College of Medicine said contact tracing was becoming impossible.

"The cases are rising so rapidly, that we cannot even do contact tracing anymore. I don't see how it's possible to even do that," he told CNN.

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The World Health Organisation says contact tracing is the process of identifying, assessing, and managing people who have been exposed to a disease to prevent onward transmission.

It's an essential public health tool for controlling infectious disease outbreaks.

There are now more than 3 million confirmed infections in the United States and 133,000 deaths.

About 905,000 people have recovered from the virus.

US hospitals are running out of ICU beds as healthcare systems in several key states buckle under a surge of new coronavirus cases after the July 4 Independence Day weekend.

A wave of new Covid-19 cases in states in the country's south and west is threatening to collapse health systems already struggling with a surge in patient numbers, the Financial Times reported.

Intensive care units in parts of Texas and Arizona are operating near full capacity, with Houston reporting hospitals were "underwater" with the explosion in cases.

The ICUs at four hospitals in Florida's Tampa area were at maximum capacity over the last weekend.

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At the same time in California, new coronavirus cases soared, stressing some hospital systems in several counties.

The sickest Covid-19 patients require precious and increasingly elusive ICU beds including ventilators, round-the clock care and specially-trained staff.

More than 15,000 patients are being treated for coronavirus in hospitals in California, Texas, Florida and Arizona, according to the FT, which said the case surge was the forerunner to a death spike.

The states most in trouble are those which reopened their economies relatively early.

US state officials and hospital management are desperately making emergency contingency plans.

Californian Governor Gavin Newsom announced the number of people hospitalised with Covid-19 had increased by 50 per cent over the past two weeks to about 5800.

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A third were in Los Angeles County, with 630 requiring intensive care, and new cases were increasingly in the 18 to 40 years age group, which had flouted distancing advice.

Nearly 1400 inmates at San Quentin State Prison north of San Francisco are infected.

Medical personnel prepare to test hundreds of people lined in Phoenix, Arizona, which has almost all ICU beds in the state filled. Photo / AP
Medical personnel prepare to test hundreds of people lined in Phoenix, Arizona, which has almost all ICU beds in the state filled. Photo / AP

The Tampa Bay Times reported that on top of four area hospitals having run out of ICU beds, a further four were running out, with one having just 3 per cent vacant capacity.

Florida reported 11,458 new coronavirus cases on Saturday, a new single-day record.

By Sunday, it had a statewide total of 200,111 coronavirus cases and 3731 deaths, however, state Governor Ron DeSantis refused to reimpose stricter conditions.

Following an alarming rise in cases, the previously so-called "fly over state" Arizona reported on Monday that 88 per cent of its ICU beds were occupied and it had a statewide shortage of ICU nurses.

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Across the United States, the number of cases has risen over three million, with 132,981 deaths, both figures reckoned to be conservative estimates.

The biggest number of cases per state is still led by New York, with 398,000, followed by California with 272,000, then Florida, Texas and New Jersey.

In Texas, one of the first states in the nation to ease social distancing rules, the number of patients hospitalised in Houston has quadrupled since May 31.

United Memorial Medical Center in Houston, Texas, a renowned facility with the largest agglomeration of hospitals and research laboratories in the world, is on the verge of being overwhelmed by the coronavirus cases exploding in the state.

As of Tuesday, more than 3000 people were hospitalised for the coronavirus in the region, including nearly 800 in intensive care, the Texas Tribune reported.

It quoted one expert saying, "it's time to be alarmed".

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