An early Omicron cluster in the U.S. suggests the variant has a shorter incubation period.

Days before the United States had its first confirmed case of the Omicron variant on Dec. 1 in California, health officials in Nebraska began looking into six probable cases of coronavirus infection in one household, among them a 48-year-old unvaccinated man who had recently returned from a conference in Nigeria.

On Dec. 2, the Nebraska Public Health Laboratory identified the Omicron variant as the cause in all six people, who ranged in age from 11 to 48. A study of the group released on Tuesday by the Centers for Disease Control and Prevention suggested that Omicron may have a shorter incubation period than previous variants: about 72 hours.

It may take as little as three days for people to develop symptoms, become contagious and test positive, compared with four to six days with Delta infections and those caused by original coronavirus, the authors observed.

Only one member of the household was fully vaccinated, and all but one member, including the traveler, had experienced confirmed infections in 2020. No household members reported underlying medical conditions.

The study also reported that the family members experienced mild symptoms. The six described their illnesses as similar to, or milder than, those experienced during their first infection.

“It is unknown whether the mild clinical syndromes or differing symptom descriptions are a result of existing immunity or altered clinical features associated with Omicron infection,” the study’s authors wrote.

“The five reinfections, including one after full vaccination, might be explained by waning immunity, the potential for partial immune evasion by Omicron, or both.”

A study in Norway that examined a large cluster of individuals who were infected with the variant at a Christmas party in Oslo also suggested that the incubation period was around three days. It is not yet clear how long people remain infectious with Omicron.

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