“We know we have several dozen cases and we’re following them closely. And we are every day hearing about more and more probable cases so that number is likely to rise,” CDC Director Dr Rochelle Walensky told ABC News in an interview, adding more data is still needed.

At least 15 states have reported Omicron cases: California, Colorado, Connecticut, Hawaii, Maryland, Massachusetts, Minnesota, Missouri, Nebraska, New Jersey, New York, Pennsylvania, Utah, Washington and Wisconsin, according to a Reuters tally.

Louisiana has also reported a probable case, while the person reported in New Jersey had also previously travelled to Georgia, which also reported the positive test.

Still, Walensky said Delta accounted for most cases in the US and as vaccines are known to protect against hospitalisation and death, health officials hope they will blunt the impact of Omicron as well.

She said that of the 90,000 to 100,000 cases a day, 99.9% of them are the Delta variant.

“We have an issue right now in the US about Delta,” Walensky told ABC’s “This Week” programme.