Vials of the Pfizer coronavirus vaccine, which were used to vaccinate Alameda County law enforcement officials at the Alameda County Sheriff's Department's Regional Training Center in Dublin, Calif. on February 18, 2021. (Eli Walsh/Bay City News)

Fully vaccinated Alameda County residents are advised to continue to wear masks if they don’t know whether the people around them are vaccinated, county public health officials said Friday.

The advice comes following guidance Thursday from the U.S. Centers for Disease Control and Prevention saying fully vaccinated people can go without a mask in most situations, indoors or outdoors.

County officials are particularly concerned when residents are in crowded places. Alameda County is currently following the state’s guidance on mask use, which has not changed since Thursday’s CDC announcement, state officials said Friday.

“COVID-19 continues to circulate at moderate levels in our communities and more than half of Alameda County residents, including children, are not fully vaccinated,” county spokesperson Neetu Balram said.

Balram added that at work, employers must follow California Division of Occupational Safety and Health standards, including those on face coverings. The county’s rules cannot be less restrictive.