If You Have a Skin Reaction to the Moderna COVID-19 Vaccine, Don't Panic

In a letter to the editor published in The New England Journal of Medicine (NEJM) on March 3, 2021, researchers reported on a dozen people who developed skin redness, skin thickening and swelling, tenderness, or some combination of these symptoms at the injection site between four and 11 days after receiving their first dose of the Moderna COVID-19 vaccine.

Another report published in the Journal of the American Medical Association (JAMA) Dermatology on May 12, 2021 described 15 cases of skin reactions—sometimes called "COVID arm"—that occurred within two to 12 days after a first dose of the Moderna vaccine.

Among those who had a skin reaction after the first dose, 11 people also had a similar reaction after their second dose. There was also one person who had a delayed reaction after the second dose only.

While immediate reactions to the COVID-19 vaccines are common, these reports raised awareness that delayed skin reactions can also show up days to weeks after a Moderna shot.

This article will explore what skin reactions to the Moderna COVID-19 vaccine look like, how common they are, and what to do if you get one.

What Do the Reactions Look Like?

The delayed skin reactions described in the reports often resembled the raised, red welts that are caused by an insect sting. However, the reactions' appearances can vary.

The JAMA Dermatology report described the reactions as itchy, painful, and swelling pink plaques at or near the injection site.

Kimberly Blumenthal, MD, lead author of the NEJM paper and co-director of the clinical epidemiology program in the division of rheumatology, allergy, and immunology at Massachusetts General Hospital, tells Verywell that the skin reactions can be “as large as almost 20 centimeters, taking up most of the upper arm.” However, the areas can also be much smaller.

How Long Does the Reaction Last?

While usually short-lived, these skin reactions can cause significant discomfort and “are likely to generate concerns among patients and requests for evaluation,” the researchers wrote in the NEJM letter, recounting a case in which a reaction was mistaken for the bacterial infection cellulitis and medicated with antibiotics.

Antibiotics are medications that kill bacteria. Cellulitis is a bacterial infection within deep layers of skin.

There was also someone whose reaction was mistaken for cellulitis and treated with antibiotics noted in the JAMA Dermatology report of patients at Yale New Haven Hospital.

Given the considerable potential for confusion, the researchers called for increased awareness of these skin reactions to help avoid needless diagnostic testing and treatment.

Unlike cellulitis that tends to progress, or worsen over time, symptoms of skin reactions to COVID-19 vaccines tend to not progress, according a few case reports of vaccine reactions misdiagnosed as cellulitis that were published in the Journal of Primary Care & Community Health.

How Common Are the Reactions?

The reactions were first observed during a phase 3 clinical trial of the Moderna vaccine.

In a study published in the NEJM in February, 2021, the investigators reported that 12,765 (84.2%) of the 15,168 participants who had received the vaccine rather than the placebo developed an immediate skin reaction after receiving their first dose of the vaccine.

By contrast, 244 (0.8%) developed a delayed skin reaction—defined as a skin reaction that appears eight or more days later—after getting their first dose, and 68 (0.2%) developed a delayed skin reaction after getting their second dose.

A survey of more than 40,000 healthcare employees at Mass General Brigham who received either the Moderna or Pfizer COVID-19 vaccines, found that 1.9% reported itching, rash, hives, or swelling after the first dose.

Among people who had a skin reaction after the first dose and completed a survey after their second dose (609 people), 83% reported no skin reaction after their second shot.

Reactions Could Be More Common Than Statistics Show

Rebecca Saff, MD, PhD, another lead author of the NEJM report and allergy fellowship director of the allergy and clinical immunology unit in the division of rheumatology, allergy, and immunology at Massachusetts General Hospital, tells Verywell that in the trial, "adverse events, including local adverse symptoms, were solicited for the seven days after the vaccine was given and unsolicited for 28 days after the vaccine, meaning that many of the more mild reactions may have been missed if they were after day seven."

In the NEJM letter, the researchers described how and when the skin reaction manifested in people between the ages of 31 and 61—the majority of whom were White and female.

The authors explained that the small sample size limited their ability to identify any differences in the appearance of the reaction between races and ethnicities. 

“We saw the reactions on different skin tones but did not see any reaction on dark skin," Saff says. 

The JAMA Dermatology report included a broader age range of 25 to 89, with the majority of people also identified as White and female.

The authors noted that it's possible that redness gets overlooked or may not be as obvious on dark skin. The study was also limited to a single medical center during a short period of time.

Why Do the Skin Reactions Happen?

Based on the results of skin samples, or biopsies, in both the NEJM and JAMA Dermatology reports the researchers' best guess is that the skin reactions are likely caused by delayed T-cell-mediated hypersensitivity—an intense immune response that begins a minimum of 12 hours after contact with a pathogen or allergen, meaning an infectious or irritating substance.

T cells are a part of the immune system that directly attacks threats and help protect you from infections.  

“We have a lot of things that are delayed hypersensitivity, like eczema, contact dermatitis, common antibiotic rashes," Blumenthal says. "There may be genetic factors at play—we know that genetics can be a risk for certain T-cell reactions to drugs—but otherwise, we don’t fully understand why someone has a response while another does [not] for most drugs and vaccines."

What to Do if You Have a Reaction

Worry over having a skin reaction should not deter you from getting a COVID-19 vaccine. If you do develop one, don't panic. While it may cause you discomfort, it's likely not serious and should resolve on its own in a short time (about two to 11 days, according to the NEJM study).

If it worsens after appearing or isn't going away, seek medical care.

If you’re finding the symptoms hard to bear, Saff recommends several basic home remedies and over-the-counter (OTC) medications for relief. Specifically, “we would recommend taking an antihistamine such as fexofenadine or cetirizine and using a topical steroid for the itch, swelling, and redness," Saff says. "Ice can also be helpful for the swelling and pain at the site."

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.