Puseletso Lesofi, prepares to sequence COVID-19 omicron samples at the Ndlovu Research Center in Elandsdoorn, South Africa Wednesday Dec. 8, 2021. Credit: Jerome Delay / AP

The first omicron cases in the U.S. were detected mostly in vaccinated people who experienced mild illnesses, with only one hospitalization and no deaths reported so far, the U.S. Centers for Disease Control and Prevention said.

As of Dec. 8, the U.S. had investigated 43 cases of the COVID-19 variant that set the world on high alert last month because of mutations that may make it spread more easily.

About four-fifths of those cases occurred in people who had been fully vaccinated, the CDC said on Friday, including one who was in the hospital for two days. Common symptoms included cough, tiredness and congestion or runny nose, according to the agency’s Morbidity and Mortality Weekly Report.

Scientists are still trying to understand how omicron compares with the dominant delta variant. Among the big questions are how quickly omicron spreads, how well it evades immune responses and how sick it makes people. The CDC report, while early, is the first detailed look at omicron cases in the U.S.

“Many of the first reported cases of omicron variant infection appear to be mild, although as with all variants, a lag exists between infection and more severe outcomes,” and vaccination and prior COVID-19 illness can mitigate disease severity, according to the CDC report.

The authors cautioned against drawing broad conclusions from the early numbers, including because those who have recently traveled internationally may be more likely to be young and vaccinated. The majority of the reported omicron cases were among adults under the age of 40, who tend to have milder cases of COVID.

About a third of the early U.S. patients had also received another shot or booster dose, though several received that extra dose less than two weeks before becoming ill. About 14% of the omicron cases, meanwhile, had evidence of a previous COVID infection.

Omicron cases had been reported across 22 states as of Dec. 8, according to the CDC. Additional omicron infections from late last month are likely to emerge soon, the authors said. Of the 43 cases, the earliest reported symptoms occurred in mid-November, the report said.

Emma Court, Bloomberg News