Highly contagious variants of COVID-19 are causing a new wave of cases in the United States as people return to something akin to normal living amid fewer social restrictions.

The Omicron sub-variants BA.5 and BA.4, which have become dominant among new coronavirus cases, are fueling the rise in infections and hospital admissions nationwide. The surge comes as more people return to their workplaces and take part in indoor activities.

"With increasing rates of transmission associated with highly infectious sub-variants that evade some of the previous acquired immunity from vaccines and previous COVID infections, we are all dealing with elevated risk," Los Angeles County Public Health Director Barbara Ferrer said on Friday.

The region logged 6,416 new cases and 18 new deaths on Friday, the highest number since March 30.There were 1,021 patients with the virus in hospital on Friday, the highest since Feb 26.

Ferrer cited "high levels of community transmission over the past month" as a contributing factor to the rise in the number of hospital admissions.

"We all now face a higher risk of getting COVID because there are more people infected with the virus who can transmit to others when we gather, go shopping or attend events. Essential workers, especially those that work directly with the public, also now have more exposures to the virus during their workday."

In total, the region has reported 3,160,032 cases and 32,413 deaths since the start of the pandemic.

Testing capacity cut

In New York, too, the number of infections is rising again.

"We are in a moment of high transmission," the New York City Health Commissioner Ashwin Vasan told ABC News on Wednesday. "If you are going indoors, wear masks."

On Friday New York public health officials reported a 15.4 percent COVID-19 positivity rate with a daily average of 3,541 cases. It is unknown whether the city is plateauing or "if this is the beginning of a more prolonged disease", Vasan said.

The Omicron sub-variants are behind the rise in cases and hospital admissions in places where BA.4 and BA.5 have been dominant for weeks or months, despite some immunity among people who had a prior infection, The New York Times reported.

Experts are concerned that all positive cases are not being accounted for at a time when the number of infections is rising again.

The country's public testing capacity has been significantly reduced as testing sites run by state and local governments have wound down, and more states have stopped giving daily data updates, the Times said.

Laboratory-based PCR testing capacity this month will be only half of what it was in March, according to an estimate by Health Catalysts Group, a research and consulting firm.