Universities in California are reinstating indoor mask mandates amid a surge in Covid cases fuelled by the hyper-transmissible Omicron BA.5 variant.

The University of California Irvine announced it was reimposing compulsory mask in campus buildings from Monday regardless of vaccination status, the Los Angeles Times reported.

The move came after Orange County health officials shifted to the “high” transmission rate  according to the Centers for Disease Control and Prevention tracker.   

Other University of California campuses including UCLA have also ordered staff and students to wear masks in all indoor spaces, reinstated weekly monitoring of symptoms and strongly recommend weekly testing.

UC Riverside requires the non-vaccinated to wear masks inside, and vaccinated to wear face coverings in “high-risk” situations.

San Diego Unified has also brought back an indoor mask mandate from Monday after the county health department also entered the “high” transmission level on Thursday.

Ninety per cent of the California population are living in counties reporting high transmission rates, which the CDC recommends should force universal masking indoors.

The CDC reported a total of daily average of 126,023 new cases on Friday. Hospitalisations have doubled since May to more than 5,000 new patients being admitted every day.

The BA.5 Omicron variant is the most infectious strain seen so far, and is effective at evading protection from vaccines. Experts say it does not appear to be more severe than previous variants.