CSU, which is the largest four-year public university in the nation and has 23 campuses, posted the mandate Wednesday on its website. All those affected must receive their booster shots by February 28, although the school will accept medical and religious exemption requests.

“Vaccination, including a booster when eligible, remains our most effective strategy against infection and severe disease,” said Chancellor Joseph Castro.

“This is particularly important in light of the rapid rise of cases of COVID-19 throughout the state and nation as the Omicron variant spreads. Implementing the booster requirement now will help mitigate the potential spread of the variant on campuses as they repopulate in January after the winter break,” Castro said.

The mandate was put into effect immediately after its announcement. Faculty and staff represented by unions will not be affected until the school is finished bargaining with their respective unions. Once these negotiations are finished, the revised university vaccination policy will be released. Although the mandate affects the entire university, individual colleges can make earlier deadlines if necessary.

CSU’s student population is around 485,000, with 55,000 faculty and staff members.

Much about the Omicron coronavirus variant remains unknown, including whether it causes more or less severe illness. Scientists say Omicron spreads even easier than other coronavirus strains, including Delta. Early studies suggest the vaccinated will need a booster shot for the best chance at preventing an Omicron infection, but even without the extra dose, vaccination still should offer strong protection against severe illness and death.

The announcement comes a day after the University of California announced a similar mandate for its 10 campuses, saying that a COVID-19 booster shot is required under the existing UC policy that says students and staff must keep their vaccination status up to date.

UC Chancellor Michael Drake outlined the policy in a letter to chancellors Tuesday. Several UC campuses have announced that classes will begin remotely at the start of the new term, including UCLA, UC Irvine, UC Santa Cruz and UC San Diego. The delays of in-person classes range from one to two weeks, which campuses say will allow them to conduct extensive testing and reduce the risk of spreading illness after the winter recess.

The Associated Press contributed to this report.