People are getting 'COVID nails,' and one expert says the unusual lines could be as useful as an antibody test to prove previous infection

• A horizontal line across the nail could be a sign of a previous COVID-19.
• If "COVID nails" were an infection marker it could be as useful as an antibody test, Prof Spector said.
• The lines can happen on the fingers or toes and are harmless.

Odd-looking nails could be a sign of previous coronavirus infection, a top UK scientist has said.

Horizontal lines across the nail that appear several months after catching coronavirus can happen in both fingers and toes, Tim Spector, professor of genetic epidemiology at King's College, told Insider.

Spector, who leads the world's largest coronavirus symptom study, said that he'd received multiple anecdotal reports of so-called COVID nails from people using the ZOE COVID Symptom Study app, a symptom tracker with more than 4 million users globally.

Spector's hunch is that this isn't a random phenomenon as it can happen after other infections too.

The underlying theory is that a stress line forms after the body shuts down for a brief period of time to fight the infection. During this time period nails don't grow. It also can happen after other stresses like chemotherapy and severe malnutrition.

"It's like a mark on a tree when an event happened," Spector explained.

Dr. Tanya Bleiker, president of the British Association of Dermatologists, told HuffPost UK that dermatologists have seen COVID-19 patients with horizontal lines on their nails too.

"These changes have long been recognized as 'Beau's lines' and are transverse indents in the nail of many, or all, fingernails and sometimes toenails," she said.

Beau's lines are not harmful themselves and usually grow out after several months.

Red, moon shaped lines across the nail are another coronavirus nail manifestation reported by dermatologists in August.

Read more: How coronavirus variants called 'escape mutants' threaten to undo all our progress

A cheap alternative to an antibody test

Spector said that it's not yet clear whether the nail changes are linked to disease severity for coronavirus. With other infections, the more severe the disease, the more likely you are to have a marker on your nail, he said.

"It may be a marker of severe infection [for COVID-19], but it would be more helpful if it wasn't," Spector added.

Spector explained that if nail changes were a marker of previous coronavirus infection then it could help people figure out whether they've had COVID-19 before without the need for a test that can be invasive and expensive.

"If we get enough numbers that are associated with asymptomatic COVID-19, that's a cheap antibody test," he said. "People just need to look down at their nails."