(The Hill) — The Centers for Disease Control and Prevention (CDC) detected 43 COVID-19 cases attributed to the omicron variant, nearly all of which were considered mild and discovered in the first week of December.

At least 22 states documented at least one COVID-19 case associated with the new variant of concern during the first eight days of December, according to data released by the CDC on Friday. Out of the 43 cases, the agency confirmed one hospitalization, which lasted two days, and zero deaths. 

While the U.S. confirmed its first case of omicron last week in a California resident, the CDC said at least one patient who traveled internationally developed symptoms on Nov. 15. The timeline means the omicron strain was likely in the U.S. longer than initially thought, as the California patient first experienced symptoms on Nov. 25.

South Africa first reported the omicron strain to the World Health Organization (WHO) on Nov. 24 — two days before the international organization designated it as a variant of concern. 

CDC Director Rochelle Walensky detailed some of the data in an interview with The Associated Press on Thursday, including that most of the 43 cases so far were considered mild and that most of the patients had been vaccinated. 

Almost 80% of the patients with an omicron strain had a full primary series of a COVID-19 vaccine. Fourteen had received an additional or booster dose, including five who got that extra dose less than 14 days after symptoms emerged. 

One-third of the patients had traveled internationally within 14 days of developing symptoms or testing positive. Most of the infected individuals ranged from 18 to 39 years old, although the strain also infected four children and four adults age 65 and older.

Only three of the patients were asymptomatic, with the most common symptoms reported being cough, fatigue, congestion and runny nose. Six patients had previously contracted COVID-19.

But the CDC researchers warned that “even if most infections are mild, a highly transmissible variant could result in enough cases to overwhelm health systems.”

The delta variant is still considered the most prevalent strain in the country by far, estimated to make up 99.9% of cases across the U.S. last week. 

In response to the omicron variant’s emergence, the U.S. initiated a travel ban for eight African countries near South Africa and Botswana, which were the first countries to report the strain. The CDC also started a voluntary airport-based genomic surveillance program in four airports. 

The agency updated its guidance to recommend booster doses for all adults in the days after the omicron variant was reported. On Thursday, the CDC expanded booster access to 16- and 17-year-olds.

In its report, the CDC called for the U.S. to use prevention strategies including vaccinations, masking, improved ventilation, testing, quarantine and isolation to curb the spread of the virus, including cases involving the omicron strain.