California State University Chancellor Joseph L. Castro announced Thursday that students, faculty and staff in America’s largest four-year public university system can only return to campuses this fall if they are vaccinated against COVID-19.

The plan will take effect once the Food and Drug Administration has approved one or more of vaccines. The two vaccines being administered across the U.S., Pfizer and Moderna, were authorized by the FDA for emergency use. While the FDA did authorize the Johnson & Johnson vaccine for emergency use, the federal agency paused the use of the vaccine after six recipients developed rare blood clots.

This is a shift away from Castro’s previous vaccination policy. In a press conference last February, Castro stated that the CSU system would be unable to enforce a vaccination requirement due to legal difficulties. Castro said that although they would have been unable to require vaccinations, the CSU was “going to do [its] very best to inspire participation and to keep everyone as safe as possible on the campuses.”

In a press release published today, the CSU system stated that before amendments are made to the existing immunization requirements it will “Engage the California State Student Association, the CSU Academic Senate and labor union,”.

The new directive will be implemented in the CSU at the beginning of the fall 2021 semester or when a vaccine receives approval, whichever date arrives sooner, according to their statement.

“Together, the CSU and UC enroll and employ more than one million students and employees across 33 major university campuses,” said Castro. “So this is the most comprehensive and consequential university plan for COVID-19 vaccines in the country.”

Every California resident is now eligible to receive a COVID-19 vaccine and nearly half of all Californians have received one dose of the vaccine.