The most common symptoms of COVID-19 are fever, dry cough and loss of taste and smell. Other signs frequently seen include headaches, muscle and joint pain, nasal congestion and fatigue.
A less common symptom is rashes in several ways. These were slower to report, partly due to the wide variety that appeared in patients with COVID-19, making it more challenging to establish a consistent correlation.
However, it is important to know how COVID-19 affects the skin. A recent study found that for 17% of COVID-19 patients with multiple symptoms, skin rashes were the first symptom to appear, while for 21% of patients the rash was the only symptom.
Being able to identify the effects of COVID-19 on the skin can allow cases to be detected earlier – or even detected completely in people who are asymptomatic. This can help to limit transmission.
With that in mind, here are the four main types of skin changes to watch for and the possible reasons why they occur.
Lesions similar to chilblains
They are red, swollen or blistered skin lesions that mainly affect the toes and soles of the feet, colloquially known as “COVID toes”. Over the course of one to two weeks, the lesions will become even more discolored and flatten out and, after that, will resolve spontaneously without treatment.
A substantial number of these injuries have been observed, especially in adolescents and young adults with no or only mild symptoms of COVID-19. They constitute the majority of skin problems associated with the virus. In two international reports on different types of suspected COVID-related skin diseases, about 60 percent of patients with skin complaints reported these injuries.
However, as these lesions correlate with mild illness, many of the patients with them in these studies did not qualify for a COVID-19 test at the time, and 55 percent were asymptomatic.
Therefore, although the rapid increase in these injuries 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 some other related factor.
Exactly when they appear is also unclear. In a study that analyzed 26 patients with suspected skin changes related to COVID, 73% had lesions similar to those of chilblains. None of the patients had respiratory symptoms and were all COVID negative at the beginning of the lesions. One explanation is that these lesions appear only after a long delay – up to 30 days after infection.
The cause of these injuries has been debated. A possible culprit may be type 1 interferons, proteins that regulate the antiviral properties of the immune system.
The theory is that the high production of these interferons can result in the rapid elimination of coronavirus by patients, but it can also cause damage to blood vessels and increase inflammation. This would explain the coincidence of mild or nonexistent illness, negative tests and skin damage.
Another theory concerns ACE2, the molecule that the coronavirus uses to enter cells. It is present in many types of cells, including sweat glands, common on the palms and soles of the feet. This can make these areas particularly vulnerable to damage from the virus.
Or it could be that the damage to blood vessels, caused by the immune response or by the virus, leads to cell death and several mini blood clots in the toes.
Macculopapular rash
This term describes flat, raised areas of discolored skin. A study of 375 patients in Spain found that 47% of patients with COVID-related skin changes had this type of rash.
These were associated with more severe symptoms of COVID-19 and were found mainly in the trunk in middle-aged to elderly patients. They tend to last 7 to 18 days, appearing 20 to 36 days after infection.
One suggested cause is the body’s immune system going into activity. In some patients, a hyperinflammatory phase occurs 7 to 10 days after infection, which leads to tissue damage and potentially more serious illness and death.
Urticaria
Also known as hives, they are raised areas of itchy skin. In a study involving four hospitals in China and Italy, 26 percent of patients with COVID-19 who complained of skin changes had hives.
Hives usually precede or appear at the same time as other symptoms, making them useful for diagnosis. They are more common among middle-aged patients and are associated with more serious illnesses. Viral infections are a known trigger of hives, as they cause cell degradation and the release of histamine through a cascade of reactions in the immune system.
However, it is important to remember that urticaria is also an observed side effect of many drugs used to treat COVID-19, such as corticosteroids and remdesevir.
Vesicular lesions
These are clear bags filled with liquid under the skin, similar to those seen in chicken pox. They are less common compared to the skin diseases above: in the Spanish study mentioned earlier on skin changes associated with COVID-19, only 9 percent of patients had these vesicles.
However, they are considered a more specific indication of someone with COVID-19 than those already listed and are therefore more useful for diagnosis. They appear to manifest in patients with mild illness around 14 days after infection.
They are believed to be caused by prolonged inflammation, with antibodies attacking the skin and damaging its layers, resulting in fluid-filled pockets. 
Vassilios Vassiliou, Senior Clinical Professor in Cardiovascular Medicine, University of East Anglia and Subothini Sara Selvendran, Visiting Researcher in Medicine, University of East Anglia.
This article was republished from The Conversation under a Creative Commons license. Read the original article.