As the coronavirus pandemic spread across the world this year to such devastating effect, many of us were asking the same questions. How does the virus spread? How do I protect myself from the infection?
The truth is, we're still learning how SARS-CoV-2 works. Official guidance from the CDC suggests the main way the virus spreads is through respiratory droplets or small particles, ejected from the mouth or nose of infected people, and then inhaled by others.
But that's not the only way the virus circulates. The same infectious droplets and particles can land on surfaces and be transferred by touch – meaning infection could result if you touch something with virus particles on it, and then touch your mouth, nose, or eyes, the CDC says.
While this general advice is repeated by health authorities the world over, there's still a lot we don't know about how the coronavirus might enter the body through the eyes, although scientists suggest it's "biologically plausible".
However, new evidence suggests at least some of the eye may in fact be resistant to SARS-CoV-2 – even while it's susceptible to other kinds of viruses.
In a new study, researchers at Washington University in St. Louis found that the cornea – the transparent dome at the front of the eye, which covers the iris and pupil – appeared to be resistant to coronavirus infection in experiments, although they're eager to emphasise the findings are only preliminary.
"Our findings do not prove that all corneas are resistant," says molecular microbiologist Jonathan J. Miner, the first author of the study.
"But every donor cornea we tested was resistant to the novel coronavirus. It's still possible a subset of people may have corneas that support growth of the virus, but none of the corneas we studied supported growth of SARS-CoV-2."
In experiments using corneal tissue from 25 human donors and also mice corneas, the researchers exposed the eye tissue to three separate viruses: SARS-CoV-2, Zika virus, and herpes simplex virus 1 (HSV-1, which produces cold sores).
In the human cornea explants tested (which also contained some conjunctiva tissue, the membrane that covers the rest of the front of the eye), the experiment showed that herpes and Zika virus were able to replicate in the tissue – but tests showed no sign of SARS-CoV-2 replication.
"The cornea and conjunctiva are known to have receptors for the novel coronavirus, but in our studies, we found that the virus did not replicate in the cornea," says senior author and ophthalmologist Rajendra S. Apte.
"Our data suggest that the novel coronavirus does not seem to be able to penetrate the cornea."
As for how the human cornea and conjunctiva might be capable of resisting SARS-CoV-2, the team isn't entirely sure. A potential molecular inhibitor of viruses in the eye – called interferon lambda – was able to limit virus growth in the human cornea for HSV-1 and Zika virus, but blocking the protein didn't seem to boost SARS-CoV-2's ability to replicate.
Without more to go on, the researchers' best guess for now is that the human cornea's resistance to coronavirus is "likely regulated by a distinct antiviral pathway". Quite what that pathway is we still don't know, and the team says further study is needed to confirm these findings.
In other words, health professionals shouldn't ditch their protective eyewear yet, and until we know otherwise, nobody should assume coronavirus can't get into the body via the eyes, despite the cornea's seeming resistance.
"It's important to respect what this virus is capable of and take appropriate precautions," Miner says.
"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."
The findings are reported in Cell Reports.