The most common symptoms of COVID-19 are fever, dry cough, and loss of taste and smell. Other symptoms commonly seen are headaches, muscle and joint pain, nasal congestion, and fatigue.
A less common symptom is skin rashes in various forms. These were reported more slowly, in part because of the wide variety that has occurred in COVID-19 patients, making it more challenging to establish a consistent correlation.
Still, it is important to know how COVID-19 affects the skin. A recent study found that in 17 percent of COVID-19 patients with multiple symptoms, a rash was the first symptom to appear, while in 21 percent of patients, a rash was the only symptom.
By being able to identify the effects of COVID-19 on the skin, cases can be noticed earlier – or even noticed altogether in people who are otherwise asymptomatic. This could help limit transfer.
With that in mind, here are the four main types of skin changes to watch out for and the possible reasons why they occur.
Chilblain-like lesions
These are red, swollen, or blistered skin that primarily affects the toes and soles of the feet, popularly known as “COVID toes”. Over the course of one to two weeks, the lesions will discolor and flatten even more, then resolve spontaneously 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 related to the virus. In two international reports on different types of suspected COVID-related skin conditions, approximately 60 percent of patients with skin complaints reported these lesions.
However, since these lesions correlate with mild disease, many of the patients with them in these studies were not eligible for a COVID-19 test at the time, and 55 percent were otherwise asymptomatic.
So while the rapid emergence of these lesions during the pandemic suggests they are related to 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 somewhat unclear. In a study analyzing 26 patients with suspected COVID-related skin changes, 73 percent presented with chilblain-like lesions. None of the patients had respiratory symptoms and all were COVID negative at the onset of their lesions. One explanation is that these lesions appear only after a long delay – up to 30 days after infection.
The cause of these lesions has been discussed. A possible culprit are 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 clear up the coronavirus quickly, as well as cause damage to blood vessels and increased inflammation. This would explain the coincidence of mild or nonexistent disease, negative tests, and skin damage.
Another theory concerns ACE2, the molecule that the coronavirus uses to get into cells. It is present on many types of cells, including those in the sweat glands, which are common on the palms of the hands and 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 the immune response or the virus, leads to cell death and multiple mini-blood clots in the toes.
Maculopapular rash
This term describes both flat and raised areas of discolored skin. A study of 375 patients in Spain found that 47 percent of patients with COVID-related skin changes had this type of rash.
These were associated with more severe COVID-19 symptoms and were mainly found on the trunk in middle-aged to elderly patients. They tended to last for 7-18 days and appeared 20-36 days after infection.
One possible cause is that the body’s immune system goes into overdrive. In some patients, a hyperinflammatory phase occurs 7-10 days after infection, leading to tissue damage and possibly more serious illness and death.
Hives
Also known as urticaria, these are raised areas of itchy skin. In a study involving four hospitals in China and Italy, 26 percent of COVID-19 patients who complained of skin changes developed hives.
Hives usually precede or present with other symptoms, making them useful for diagnosis. They are more common in middle-aged patients and are associated with more serious illness. Viral infections are a known trigger of hives because they cause the breakdown of cells and the release of histamine through a cascade of reactions in the immune system.
However, it’s important to remember that hives is also a known side effect of many drugs used to treat COVID-19, such as corticosteroids and remdesevir.
Vesicular lesions
These are clear, fluid-filled sacs under the skin, similar to those found in chicken pox. They are less common compared to the above skin conditions: In the aforementioned Spanish study of skin changes associated with COVID-19, only 9 percent of patients had these blisters.
However, they are believed to be a more specific indication of someone who has COVID-19 than those already listed, and thus are more useful for diagnosis. They appear to occur about 14 days after infection in patients with mild disease.
They are thought to be caused by long-term inflammation, in which antibodies attack the skin and damage the layers, resulting in fluid-filled sacs. 
Vassilios Vassiliou, Senior Clinical Lecturer in Cardiovascular Medicine, University of East Anglia and Subothini Sara Selvendran, Visiting Researcher in Medicine, University of East Anglia.
This article has been republished from The Conversation under a Creative Commons license. Read the original article.