The Centers for Disease Control and Prevention (CDC) revealed on Friday that most of the 43 Omicron COVID-19 cases in the United States are fully vaccinated.
The CDC added a third of them had received a booster dose before they got infected with Omicron.
Based on the CDC data, from the 43 cases attributed to the 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.
With this data, CDC has expressed concern that the current COVID-19 vaccines may offer less protection against the highly transmissible new variant.
The profile of the Omicron cases in the US is 25 were in people aged 18 to 39 and 14 had traveled internationally.
Six people had previously been infected with the coronavirus.
The CDC further revealed the symptoms of most Omicron cases in the US are only mild such as coughing, congestion, and fatigue.
Sources said the first known U.S. Omicron case was identified on Dec. 1 in a fully vaccinated person who had traveled to South Africa.
The CDC said that the earliest date of symptom onset was Nov. 15 in a person with a history of international travel.