Scientists can now explain why Covid-19 patients develop chilblain-like lesions on their toes and even fingers.
Covid toe appears to be a side effect of the body switching into attack mode to fight off the virus.
The researchers say they have pinpointed the parts of the immune system that appear to be involved.
The findings, in the British Journal of Dermatology, may help with treatments to ease the symptoms.
It can happen at any age, but affects children and teenagers more commonly. For some it is painless, but the rash can be extremely sore and itchy, with tender blisters and swelling.
The affected skin usually the toes, but sometimes the fingers can look red or purple. Some people develop painful raised bumps or areas of rough skin. There can also be pus.
Some have it for months, others for weeks.
Often, they will have none of the classic Covid symptoms, such as the persistent cough, fever and loss or change in smell or taste.
These latest study findings, based on blood and skin tests, suggest two parts of the immune system may be at play.
Both involve mechanisms the body uses to fight coronavirus.
One is an antiviral protein called type 1 interferon, and the other is a type of antibody that mistakenly attacks the person’s own cells and tissues, not just the invading virus.
Cells lining small blood vessels supplying the affected areas are also involved, say the investigators from the University of Paris, France.
Usually chilblains go away on their own. But some may need treatment with creams and other drugs.