The most common symptoms of COVID-19 are fever, dry cough, and loss of sense of taste and smell. Other signs that are seen frequently are headaches, muscle and joint pain, nasal congestion and fatigue.
A less common symptom is rashes of various forms. They have been reported more slowly, in part because of the wide variety that have appeared in patients with COVID-19, making it more difficult 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, rashes were the first symptom to appear, while for 21% of patients rashes were their only symptom.
Being able to identify the effects of COVID-19 on the skin may allow cases to be detected earlier, or even collected entirely in people who are otherwise asymptomatic. This could help limit transmission.
With that in mind, here are the four main types of skin changes that need to be considered and the possible reasons why they occur.
Chilblain-like injuries
These are red, swollen, or blistered skin lesions that mainly affect the toes and sole of the foot, colloquially known as “COVID toes”. Over the course of a week or two, the lesions will fade even more and flatten, and then resolve spontaneously without treatment.
A substantial number of these lesions have been seen, mainly in adolescents and young adults with no mild symptoms or only COVID-19. These are most of the skin problems associated with the virus. In two international reports of different types of suspected COVID-related skin conditions, about 60% of patients with skin problems reported these lesions.
However, given the lesions that correlate with a mild illness, many of the patients with these studies did not qualify for a COVID-19 test at that time and 55% were asymptomatic.
Thus, although the rapid increase in these injuries during the pandemic suggests that they are associated with COVID-19, no direct confirmation of this has been established. They may be caused by some other related factor.
When they appear is also unclear. In a study that looked at 26 patients with suspected COVID-related skin changes, 73 percent had lesions similar to the numbers. None of the patients had respiratory symptoms and all were VOCID negative at the onset of their injuries. One explanation is that these lesions only appear after a long delay, up to 30 days after infection.
The cause of these injuries has been debated. A 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 can cause patients to quickly eliminate the coronavirus, but also cause damage to blood vessels and increase inflammation. This would explain the coincidence of mild or non-existent diseases, negative tests and skin damage.
Another theory concerns ACE2, the molecule that the coronavirus uses to enter cells. It is present in many cell types, including those of the sweat glands, which are common in the palms of the hands and soles of the feet. This could make these areas especially vulnerable to damage from the virus.
Or it could be that the damage to the blood vessels, caused by the immune response or the virus, causes cell death and several mini blood clots on the toes.
Maculopapular rash
This term describes the flat, elevated 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 patients up to the elderly. They tended to last between 7 and 18 days, appearing between 20 and 36 days after infection.
One of the suggested causes is that the body’s immune system is overheated. In some patients, a hyperinflammatory phase occurs 7-10 days after infection, causing tissue damage and potentially more serious illness and death.
Beehives
Also known as hives, they are elevated areas of itchy skin. In one study that included four hospitals in China and Italy, 26 percent of COVID-19 patients complained of skin changes presented with hives.
Beehives usually precede or present 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 hives are also a prominent side effect of many medications that have been used to treat COVID-19, such as corticosteroids and remdesevir.
Vesicular lesions
These are clear, fluid-filled bags under the skin, similar to those seen in chickenpox. They are less common compared to previous skin conditions: in the aforementioned Spanish study on skin changes associated with COVID-19, only 9% of patients had these vesicles.
However, they are thought to be a more specific indication of someone having COVID-19 than those already on the list and are therefore more useful for diagnosis. They appear to occur in patients with mild disease about 14 days after infection.
They are believed to be caused by prolonged inflammation, with antibodies attacking the skin and damaging its layers, resulting in sacks full of fluids. 
Vassilios Vassiliou, senior clinical professor of cardiovascular medicine at the University of East Anglia and Subothini Sara Selvendran, visiting researcher in medicine at the University of East Anglia.
This article is republished from The Conversation under a Creative Commons license. Read the original article.