Scientists believe they can explain why some people who catch Covid develop chilblain-like lesions on their toes and even fingers.
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.
What is Covid toe?
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.
Some have it for months, others for weeks.