Most of the 43 COVID-19 cases caused by the Omicron variant identified in the United States were in people who were fully vaccinated and a third of them had received a booster dose, a US report says.

The US Centers for Disease Control and Prevention (CDC) said that of the 43 cases attributed to Omicron variant, 34 people had been fully vaccinated.

Fourteen of them had also received a booster, although five of those cases occurred less than 14 days after the additional shot before full protection kicks in.

While the numbers are very small, they add to growing concerns that current COVID-19 vaccines may offer less protection against the highly transmissible new variant.

The Omicron variant of the coronavirus has been found through testing in about 22 states so far after first being identified in southern Africa and Hong Kong in late November.

Among the Omicron cases, 25 were in people aged 18 to 39 and 14 had travelled internationally.

Six people had previously been infected with the coronavirus.

Most of them only had mild symptoms such as coughing, congestion and fatigue, the report said, and one person was hospitalised for two days.

Other symptoms reported less frequently including nausea or vomiting, shortness of breath or difficulty breathing, diarrhoea and loss of taste or smell.

The CDC said that while many of the first reported Omicron cases appear to be mild, a lag exists between infection and more severe outcomes.

Symptoms would also be expected to be milder in vaccinated persons and those with previous SARS-CoV-2 infection.

The first known US Omicron case was identified on December 1 in a fully vaccinated person who had travelled to South Africa.

The CDC said that the earliest date of symptom onset was November 15 in a person with a history of international travel.

The Delta variant still accounts for more than 99 per cent of all US cases.

But reports from South Africa suggest the Omicron variant is very transmissible.

Even if most cases are mild, a highly transmissible variant could result in enough infections to overwhelm health systems, the CDC cautioned.

Laboratory studies released this week indicate that the Omicron variant will blunt the protective power of two doses of Pfizer and BioNTech's COVID-19 vaccine, although a third dose may restore that protection.

The US has authorised COVID-19 vaccine boosters for all people in the country age 16 and older.