Prior research in human and mouse corneal tissue had demonstrated that Zika virus could be shed in tears.

The researchers wanted to learn whether the cornea might serve as an entry point for SARS-CoV-2, and they tested it by exposing the eye tissue to the different viruses and observing whether the viruses could grow and replicate.

They also identified key substances in corneal tissue that can promote or inhibit viral growth.

One inhibitor they identified is called interferon lambda. They found that interferon lambda prevented efficient growth of Zika virus and herpes simplex virus in the cornea. But with SARS-CoV-2, levels of the substance had no effect on whether the virus could replicate. It simply could not gain a foothold whether interferon lambda was present or not.

This suggests COVID-19 probably cannot be transmitted through a cornea transplant or similar procedures in the eye.

"Our data suggest that the novel coronavirus does not seem to be able to penetrate the cornea," said Rajendra S. Apte, professor of the Department of Ophthalmology and Visual Sciences at the university.

However, because of unknowns involving the tear ducts and the conjunctiva, it's too soon to dismiss the importance of eye protection, the researchers said.

"It's important to respect what this virus is capable of and take appropriate precautions," said first author Jonathan J. Miner, an assistant professor of medicine, of molecular microbiology and of pathology and immunology. "We may learn that eye coverings are not necessary to protect against infection in the general community, but our studies really are just the beginning. We need larger clinical studies to help us better understand all the potential routes of SARS-CoV-2 transmission, including the eye."

The findings were published Tuesday in the journal Cell Reports.