We’ve heard of Covid tongue, rashes and even fingers and toes – and now there’s yet another possible sign you’ve had the virus: Covid nails.

Professor Tim Spector, principal investigator of the Zoe Covid Symptom Study app, shared a photo of the phenomenon on Twitter, suggesting Covid nails are “increasingly being recognised as the nails recover after infection and the growth recovers, leaving a clear line”.

Also known as Beau’s lines, the horizontal grooves or indentations appear in the nail plate and can be caused by the interruption of growth to your nail due to injury or illness. Prof Spector noted that, in Covid patients, they can occur without the presence of skin rashes and appear to be harmless.

Case reports published in health journals have noted the phenomenon has been recorded in Covid patients elsewhere. One 45-year-old man presented with horizontal grooves on his fingernails and toenails – three and a half months previously, he’d been given a diagnosis of Covid-19 after a positive PCR swab test. His symptoms lasted for 10 days and he didn’t require admission to hospital.

Dr Tanya Bleiker, president of the British Association of Dermatologists, tells HuffPost UK it’s something dermatologists are witnessing in Covid patients too. 

“These changes have long been recognised as ‘Beau’s lines’ and are transverse indents in the nail of many, or all, fingernails and sometimes toenails,” she says.

The indents tend to appear on the fingernails between two and three weeks after an illness – and a bit later in toenails. “They are harmless and grow out with time,” adds Dr Bleiker.

It’s important to note Beau’s lines aren’t exclusive to Covid – so it’s not a surefire sign you had the virus. Other causes include trauma to the nail, eczema, severe malnutrition, Raynaud’s disease, hypertension, epilepsy, renal failure, Kawasaki disease and chemotherapy. 

They’ve also been associated with the presence of a high fever, according to Dermatology Advisor, which is one of the key symptoms of coronavirus but also many other ailments including scarlet fever, pneumonia and malaria.

There’s no specific treatment for such lines and researchers note they tend to go back to normal if the underlying condition is resolved. Once that’s happened, it will probably take about six months for the nails – and lines – to grow out and disappear fully.

If they don’t grow out, or more appear, it might be worth speaking to a dermatologist or your GP about whether another underlying condition could be causing it. 

Another nail change that appears to be linked to coronavirus is the presence of red half-moon markings on the nails near the cuticles. Researchers aren’t sure why this happens, but they believe it might be something to do with vascular inflammation caused by the virus.