Officials said the patients are exhibiting mild symptoms of the virus.
According to the Alameda County Public Health Department, the cases are among 12 local COVID cases linked to a Nov. 27 wedding in Wisconsin. Health leaders said someone had attended the wedding after taking an international trip.
Genomic sequencing was used to identify the five infected with the omicron variant of COVID. Genomic sequencing data is not yet available for all 12 cases, so health officials have not determined which variant they have.
The infected residents are between the ages of 18 and 49.
"One individual is a resident of the City of Berkeley, which has a separate health jurisdiction, and the remaining 11 are residents of Alameda County. All 12 individuals were vaccinated, and most had received boosters," the county public health department said in a statement.
No one has been hospitalized.
The latest cases of the omicron variant comes after health officials discovered the first U.S. case in San Francisco.