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MINNEAPOLIS (AP) — A growing number of professors and staff at the University of Minnesota are calling on the school to mandate COVID-19 vaccinations this fall, as cases of the virus are surging.

More than 500 people have signed a letter supporting a vaccine mandate. The University of Minnesota chapter of the American Association of University Professors said in a statement Thursday that there is “broad frustration and deep anger among faculty at Twin Cities that has been building over the summer about the unsafe reopening policies put forward by the administration.”

The Star Tribune reported that the university is among a handful of colleges in the Big Ten that are not requiring COVID-19 vaccinations. Minnesota State Colleges and Universities is also not requiring the vaccine at its schools. Vaccines are being required at nearly a dozen private colleges in the state.

The university implemented a mask mandate at its five campuses this week, but it doesn’t plan to enforce social distancing or require regular COVID-19 testing for students.

University leaders have said a vaccination mandate would be difficult to enforce because Minnesota’s immunization law has an exemption for people who hold beliefs against it. Administrators have also said a mandate could be “legally problematic” since the vaccines were authorized for emergency use but do not yet have full Food and Drug Administration approval.

On Thursday, Minnesota health officials reported five new deaths from COVID-19 and 878 new cases. Data from Johns Hopkins University shows the rolling average number of daily new cases in Minnesota has increased by 408 over the last two weeks — that’s an increase of 177.5%.