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Moderna COVID-19 vaccine: What to know about side effects

The Moderna COVID-19 vaccine is a two-dose vaccine to prevent COVID-19. This Snapshot feature looks at the possible side effects and safety recommendations associated with this mRNA vaccine.

Moderna’s COVID-19 vaccine, also known as mRNA-1273, is a two-dose vaccine. The doses are administered 28 days apart, and the vaccine trains the immune system to fight against future infections with SARS-CoV-2, the virus that causes COVID-19.

Moderna’s vaccine has authorization for use in a total of 45 countries. The Food and Drug Administration (FDA) has issued this authorization for people in the United States aged 18 or older. The vaccine has also been authorized for use across the European Union after a recommendation by the European Medicines Agency, as well as in Canada and the United Kingdom.

Clinical trial data show that the vaccine has an efficacy of 94.1% at preventing symptomatic COVID-19.

Like the Pfizer-BioNTech vaccine, the Moderna vaccine is an mRNA vaccine. While these two are the first mRNA vaccines to be authorized for use in humans, scientists have been researching this technology for many years.

An mRNA vaccine functions by providing the body with genetic information to produce viral or bacterial proteins, in this case, the spike protein found on the surface of SARS-CoV-2. These proteins trigger an immune response and the production of specific antibodies, preparing the body to fight off an infection if it comes into contact with the pathogen in the future.

The vaccine only carries the information needed to make a small part of the virus. It does not contain the SARS-CoV-2 virus, and it cannot cause COVID-19.

Enzymes in our cells quickly degrade the mRNA molecules in the vaccine once the spike protein has been made. It is not possible for the vaccine to alter the body’s genetic information.

The FDA’s fact sheet for this vaccine lists the following general side effects. Also, we have included the percentages of frequency reported in Moderna’s phase 3 clinical trial data:

  • fatigue (70%)
  • headache (64.7%)
  • muscle pain (61.5%)
  • joint pain (46.4%)
  • chills (45.4%)
  • nausea and vomiting (23%)
  • fever (15.5%)

The clinical trials found that side effects were more commonly reported after the second dose and lasted around 2–3 days.

Recipients also reported the following injection site reactions:

  • pain (92%)
  • swelling (14.7%)
  • swelling of armpit lymph nodes, specifically (19.8%)
  • redness (10%)

People who have had other authorized COVID-19 vaccines have also reported these side effects.

Among recipients of mRNA vaccines, however, there have been more frequent reports of side effects from people who have had the Moderna vaccine. This study notes that after the second dose, 82% of Moderna vaccine recipients reported injection site reactions, compared with 69% of Pfizer vaccine recipients.

There have also been reports of people experiencing a red, itchy, swollen, or painful rash at the injection site. Known as “COVID arm,” the rash can appear several days after the vaccination.

This case report describes the experiences of four people who developed COVID arm 7–10 days after receiving the first dose of the Moderna vaccine. The authors of the report note that COVID arm is uncommon, not harmful, and recedes spontaneously or with the use of steroids or antihistamines.

Allergic reactions, severe and otherwise, have been reported as possible side effects of the vaccine, as a response to certain ingredients in the vaccine.

Anaphylaxis, a severe allergic reaction, is a rare side effect of vaccination. Out of 7,581,429 Moderna vaccine doses administered, there have been 19 reports of anaphylaxis, fewer than 3 cases per million.

The vaccine fact sheet lists the following potential symptoms of a severe allergic reaction:

  • difficulty breathing
  • swelling of the face and throat
  • a rapid heartbeat
  • a rash all over the body
  • dizziness
  • weakness

Like the Pfizer vaccine, the Moderna COVID-19 vaccine contains an ingredient called polyethylene glycol (PEG), which has raised concerns regarding its potential to induce an allergic reaction.

PEG, which is commonly found in laxatives, works in the vaccine as a protective coating for the mRNA molecule as it enters cells. It is still unclear whether PEG is the cause of the reported allergic reactions or whether the concentration of PEG in the vaccine is enough to cause a reaction.

It is worth noting that a PEG allergy is extremely rare. According to the Centers for Disease Control and Prevention (CDC), anyone with a history of an allergic reaction to PEG should not get an mRNA COVID-19 vaccine and should speak to their doctor about an alternative.

Moderna reported three cases of lip or facial swelling in clinical trial participants who had previously had a dermal filler cosmetic injection. The company said that the reactions were likely related to vaccination.

A possible explanation for this side effect is that the spike protein of the SARS-CoV-2 virus, which the mRNA helps the body to produce, interacts with regions around the dermal filler and induces an inflammatory response.

The CDC report that “Infrequently, people who have received dermal fillers might experience swelling at or near the site of filler injection (usually face or lips) following administration of a dose of an mRNA COVID-19 vaccine.”

The agency says that people who have dermal fillers can have mRNA COVID-19 vaccines but that they should contact a healthcare professional if they experience any swelling after the vaccination.

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