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The most common symptoms of COVID-19 are fever, a dry cough, and loss of taste and smell. Other commonly seen signs are headaches, muscle and joint pain, nasal congestion, and fatigue.
Rashes of various forms are a less common symptom. These have been slower to be reported, in part due to the wide variety that has emerged in COVID-19 patients, making it more difficult to establish a consistent correlation.
Still, it’s important to know how COVID-19 affects the skin. A recent study found that for 17% of COVID-19 patients with multiple symptoms, rash was the first symptom to appear, while for 21% of patients, rash was their only symptom.
Being able to identify the effects of COVID-19 on the skin may make it possible to spot cases earlier – or even fully detect them in people who are otherwise asymptomatic. This could help limit transmission.
With that in mind, here are the four main types of skin changes to watch out for and the possible reasons why they occur.
Frostbite-type lesions
These are red, swollen, or blistering skin lesions that primarily affect the toes and soles of the feet, commonly known as “COVID toes”. Over the course of one to two weeks, the lesions will become even more discolored and flatten out, after which they will go away on their own without treatment.
A significant number of these lesions have been observed, mainly in adolescents and young adults with no or only mild symptoms of COVID-19. They make up the majority of skin problems associated with the virus. In two international reports on different types of skin conditions suspected of COVID, about 60 percent of patients with skin disorders reported these lesions.
However, since these lesions correlate with mild illness, many patients with them in these studies were not eligible for a COVID-19 test at the time, and 55% were otherwise asymptomatic.
So, although the rapid increase in these lesions during the pandemic suggests that they are associated with COVID-19, direct confirmation of this has not been established. It is possible that they are caused by another related factor.
It is also not clear exactly when they appear. In a study analyzing 26 patients with skin changes suspected of COVID, 73% had frostbite-like lesions. None of the patients had respiratory symptoms and they were all COVID-negative at the onset of their lesions. One explanation is that these lesions only appear after a long delay – up to 30 days after infection.
The cause of these lesions has been debated. One possible culprit could be type 1 interferons, proteins that regulate the antiviral properties of the immune system.
The theory is that high production of these interferons could cause patients to clear coronavirus quickly, but also cause blood vessel damage and increased inflammation. This would explain the coincidence of a mild or no disease, negative tests and skin lesions.
Another theory concerns ACE2, the molecule that the coronavirus uses to get inside cells. It is present on many types of cells, including those in the sweat glands, which are common on the palms of the hand and the soles of the feet. This could make these areas particularly vulnerable to damage from the virus.
Or, it could be that damage to blood vessels, caused by either the immune response or the virus, leads to cell death and multiple mini blood clots in the toes.
Maculopapular rash
This term describes both the flat and raised areas of discolored skin. A study of 375 patients in Spain found that 47% of patients with skin changes linked to COVID had this type of rash.
These were associated with more severe symptoms of COVID-19 and were mostly found on the trunk in middle-aged to elderly patients. They typically lasted 7 to 18 days, appearing 20 to 36 days after infection.
One suggested cause is that the body’s immune system is overworked. In some patients, a hyperinflammatory phase occurs 7-10 days after infection, resulting in tissue damage and, potentially, more severe disease and death.
Urticaria
Also known as hives, these are itchy areas of skin. In a study of four hospitals in China and Italy, 26% of COVID-19 patients who complained of skin changes developed hives.
Urticaria usually precedes or presents with other symptoms, making them useful for diagnosis. They are more common in middle-aged patients and are associated with more serious disease. Viral infections are a known trigger for urticaria because they cause cells to break down and release histamine through a cascade of reactions in the immune system.
However, it’s important to remember that hives are also a noted side effect of many drugs that have been used to treat COVID-19, such as corticosteroids and remdesevir.
Vesicular lesions
These are transparent bags filled with fluid under the skin, similar to those seen in chickenpox. They are less common compared to the skin conditions above: In the previously mentioned Spanish study of skin changes associated with COVID-19, only 9% of patients had these vesicles.
However, it is believed that they are a more specific indication of a person having COVID-19 than those already listed, and therefore are more useful for diagnosis. They seem to present in patients with mild illness about 14 days after infection.
They are thought to be caused by prolonged inflammation, with antibodies attacking the skin and damaging its layers, resulting in bags filled with fluid. 
Vassilios Vassiliou, Clinical Lecturer in Cardiovascular Medicine, University of East Anglia and Subothini Sara Selvendran, Visiting Fellow in Medicine, University of East Anglia.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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