WASHINGTON — Twenty-five states have identified cases of the new omicron variant of COVID-19, federal public health officials said Friday as they released new data on the first 43 U.S. cases.

Of those initial, confirmed cases, more than half were among people between the ages of 18 and 39, according to Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention.

About one-third of the infections were among individuals who recently had traveled internationally, Walensky said during a news briefing Friday. And 80%, or 34 individuals, were fully vaccinated, including some who had recently received a booster.

Most of the omicron infections so far have resulted in only mild symptoms, Walensky said, adding that’s in line with what would be expected among individuals who were fully vaccinated.

Missouri reported its first case of the omicron variant on Dec. 3.  The most recent wastewater analysis report, posted Friday, shows that the delta variant is the dominant variant throughout the state.

Walensky and other public health officials said early data suggests that getting a vaccine booster could bolster protection against the new variant, urging anyone who is currently eligible — including the 16- and 17-year-olds who became eligible this week — to get a follow-up shot.

While the unknowns of the new variant still loom, officials emphasized that the highly transmissible delta variant that caused the summer surge in infections still remains the main threat.

“Over 99% of cases in this country right now are caused by the delta variant, which is driving increases in cases and hospitalizations,” Walensky said.

The seven-day average of infections nationally increased by 37% over the last week, while hospitalizations increased by 16% and deaths rose by 28% over that same time period.

In Missouri, the Department of Health and Senior Services reported 3,377 additional infections Friday and the department’s seven-day average of cases through Tuesday was 2,623 per day, up 33% from the previous week. Hospitalizations are up 22% over the same period.

Those rising numbers come after families gathered for Thanksgiving last month, and many are preparing for holiday gatherings this month.

Asked for any guidance for those wondering if they should reassess holiday travel plans, Walensky said gathering together this season will require Americans to be “vigilant” about safety precautions. She reiterated the need to ensure those getting together are fully vaccinated and boosted if possible, as well as wearing masks in the weeks leading up to any gatherings and taking a COVID-19 test.

Rudi Keller of the Missouri Independent contributed to this report.