Dec 10 (Reuters) - Most of the 43 COVID-19 cases caused by the Omicron variant identified in the United States so far were in people who were fully vaccinated, and a third of them had received a booster dose, according to a U.S. report published on Friday. The U.S. 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 traveled 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 hospitalized for two days. Other symptoms reported less frequently including nausea or vomiting, shortness of breath or difficulty breathing, diarrhea 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 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. The Delta variant still accounts for more than 99% of all U.S. cases. But reports from South Africa show that 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 suggest that the Omicron variant will blunt the protective power of two doses of Pfizer (PFE.N) and BioNTech"s COVID-19 vaccine, although a third dose may restore that protection. The U.S. has authorized COVID-19 vaccine boosterdoses for all Americans age 16 and older.