The Moderna vaccine is one of the most common vaccines that protect against SARS-CoV-2, the virus that causes COVID-19. It can cause mild side effects such as fatigue, muscle aches, and headache.

Healthcare professionals have administered over 243 million doses of the original Moderna vaccine since the start of the COVID-19 pandemic. Adults should receive two doses of the vaccine 8 weeks apart, along with appropriate booster shots.

Following the initial two doses and a first booster dose, the vaccine is highly effective at preventing severe COVID-19, hospitalization, and death. It also offers modest protection against symptomatic illness.

Although the Moderna vaccine is highly effective, like all vaccines, it can cause side effects. Typically, these side effects are mild and go away on their own within a few days.

Read on to learn more about the side effects relating to the Moderna vaccine, how long they last, and when someone should contact a doctor.

All data and statistics are based on publicly available data at the time of publication. Some information may be out of date. Visit our coronavirus hub for the most recent information on COVID-19.

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A doctor prepares a Moderna vaccine Covid-19 for an inhabitant of the ‘Home for the Elderly Kamenec’ in Ostrava, Czech Republic, on January 14, 2021. RADEK MICA/Getty Images

Most people who receive the Moderna vaccine experience mild, local side effects at the injection site. Local effects typically appear within a couple of days following vaccination and persist for around 3 days.

Systemic reactions, such as headaches, fever, and fatigue, often begin within the first 2 days of vaccination and last for 2–3 days. Overall, the side effects are generally mild and do not last long. Most people will not experience side effects after a few days.

Pain, discoloration, and swelling at the injection site are the most common side effects relating to the Moderna vaccine. These are local reactions.

Additionally, people can experience systemic side effects such as:

  • fatigue
  • headache
  • muscle pain
  • joint pain
  • chills
  • fever
  • nausea
  • swollen lymph nodes
  • COVID arm,” a skin rash at the vaccination site.

Systemic reactions are signs that the body’s immune system is building immunity to the virus. They typically peak 1–2 days after vaccination and go away within a few days.

As with other vaccines, responses to the shot can differ.

Data suggest that women often experience more side effects than men. However, research has not found a causal link, and this disparity may be because women are more likely to report side effects or certain biological differences.

People may report more frequent or severe side effects after the second dose of the Moderna vaccine compared with the first. This is likely because the second dose elicits a stronger immune response.

The Moderna vaccine is an mRNA vaccine that uses pieces of the virus’s genetic code to teach a person’s body to build immunity to the virus. Because the first dose primes the body to recognize the virus, reactions are typically mild. The second dose causes an exaggerated response.

There have been some reports of myocarditis and pericarditis following vaccination with Moderna and other mRNA vaccines. This is typically within 1 week of the second dose and is more common among young males. Although potentially serious, most people respond well to medication and rest and recover quickly.

Research also suggests that people who have had COVID-19 previously also have more severe reactions to the vaccine. This may be because their body is already primed to react to the vaccination as they have some level of preexisting immunity.

Side effects of the Moderna vaccine are similar to those of the Pfizer vaccine, another mRNA vaccine.

After getting the first dose of an mRNA vaccine, people commonly report injection site reactions such as pain, redness, and swelling after the first dose. They may report fatigue and joint pain after the second.

Individuals receiving any of the COVID-19 vaccines can expect to experience mild to moderate side effects, but these vary by age group. The Centers for Disease Control and Prevention (CDC) notes that for those aged 4–17, side effects are more common after the second dose.

Learn more about vaccine side effects in our dedicated COVID-19 hub.

Most people who experience side effects from the Moderna vaccine recover within a few days without needing medical attention. However, in rare cases, reactions can be more severe.

A person should contact a doctor if any of the following occur:

  • discoloration or swelling that continues to worsen at the injection site
  • hives
  • difficulty breathing or swallowing
  • swelling of the tongue or throat

These could be signs of a severe allergic reaction known as anaphylaxis.

Anaphylaxis is a severe allergic reaction that can be life threatening. The symptoms develop suddenly and include:

  • hives
  • swelling of the face or mouth
  • wheezing
  • fast, shallow breathing
  • a fast heart rate
  • clammy skin
  • anxiety or confusion
  • dizziness
  • vomiting
  • blue or white lips
  • fainting or loss of consciousness

If someone has these symptoms:

  1. Check whether they are carrying an epinephrine pen. If they are, follow the instructions on the side of the pen to use it.
  2. Dial 911 or the number of the nearest emergency department.
  3. Lay the person down from a standing position. If they have vomited, turn them onto their side.
  4. Stay with them until the emergency services arrive.

Some people may need more than one epinephrine injection. If the symptoms do not improve in 5–15 minutes, or they come back, use a second pen if the person has one.

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Following the Moderna shot, many people experience local reactions such as pain, redness, and swelling. Other common side effects include fatigue, headache, and muscle aches. These reactions are usually a sign that the body is building immunity.

People may experience more severe reactions after the second dose, and reactions may also be more intense in those who have previously had COVID-19.

Although side effects are common, if someone experiences difficulty breathing or swallowing, swelling of the tongue or throat, or hives after receiving the Moderna vaccine, they should seek medical attention.