A U.S. judge upheld the University of California's COVID-19 vaccine requirement against a challenge by a professor who alleged he had immunity due to a prior coronavirus infection, in what appears to be the first ruling on the issue.

U.S. District Court Judge James Selna in Santa Ana, California, said the university system acted rationally to protect public health by mandating the vaccine and not exempting individuals with some level of immunity from an infection.

More than 43 million Americans have had confirmed cases of COVID-19 and some opponents of vaccinations have argued that immunity from an infection negates the need for an inoculation.

The U.S. Centers for Disease Control and Prevention (CDC) said on Aug. 6 that a study showed vaccines offer better protection than natural immunity gained from prior infection, which wanes over time.

On Wednesday, a group of physicians who are Republican members of Congress wrote to the CDC to urge the agency to acknowledge natural immunity.

The lawmakers said if the growing number of vaccine mandates ignore natural immunity it could lead to labor shortages as people are fired for failing to get a shot. Their letter said such mandates could even trigger a security crisis because up to 20% of the military faces "separation" and many of them "likely have natural immunity."

Selna's ruling denied a motion for a preliminary injunction by Aaron Kheriaty. And while Selna said the professor at the University of California, Irvine School of Medicine did not show a likelihood of success, Kheriaty said he plans to continue the litigation.

He told Reuters he plans to use the discovery process to determine how the policy was formulated and to question the university's expert witnesses about their reasoning for rejecting his arguments on natural immunity.

The university did not immediately respond to a request for comment.