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What to know about the Moderna vaccine

A bandage, a vaccine vial and a cotton swab.

Moderna's COVID-19 vaccine was the second to be OK'd for emergency use by the U.S. Food and Drug Administration (FDA) last year. And millions of people have received it safely so far.

Here are some important questions and answers about this vaccine. (See information about the Pfizer and Johnson & Johnson vaccines.)

Q. How does the vaccine work?

A. The Moderna vaccine contains genetic material called messenger RNA (mRNA). This small piece of the coronavirus's mRNA orders the cells in your body to make copies of the distinctive but harmless spike protein that appears on the surface of the coronavirus. These spike proteins trigger an immune reaction. Your body creates antibodies, which then protect you from getting sick if you're exposed to the real virus later.

It's important to note that the vaccine doesn't contain the real coronavirus. So getting the vaccine cannot give you COVID-19.

Q. How many shots are given and how far apart?

A. This vaccine requires two shots given one month apart. Some people with weakened immune systems may need a third shot at least four weeks after the second dose to improve their response to the initial vaccine series. This shot is called an additional dose, not a booster.

Boosters are aimed at increasing the effectiveness of the vaccine. Everyone 18 and older who received the Moderna vaccine is eligible six months after their initial vaccine series. The booster does not have to be the Moderna vaccine. You can stick with Moderna if you want, or you can choose one of the other available vaccines. The Moderna booster is half the dose of the initial shots.

Q. How long after getting your shots does it take to be effective?

A. According to the Centers for Disease Control and Prevention (CDC), it usually takes a few weeks for immunity to develop after any vaccine. Trial data suggest that this vaccine starts to offer some protection within two weeks of the first shot. But you won't be considered fully immunized until two weeks after your second shot.

Q. How effective was the vaccine in clinical trials?

A. The vaccine was 94.1% effective in preventing COVID-19 in clinical trials. That's very good. FDA's benchmark was an efficacy rate of 50%.

Q. What was its safety record in clinical trials?

A. Researchers looked at safety data broken down by:

  • Age.
  • Race.
  • Ethnicity.
  • Underlying medical conditions.
  • Previous COVID-19 infections.

There were no safety concerns. Serious adverse events occurred in similar numbers among people who got the vaccine and those who got a placebo.

Q. What were the most common side effects?

A. The most commonly reported side effects were:

  • Pain at the injection site.
  • Tiredness.
  • Headache.
  • Muscle pain.
  • Chills.
  • Joint pain.
  • Swollen lymph nodes in the same arm as the injection.
  • Nausea and vomiting.
  • Fever.

These side effects were more common after the second dose.

Q. Who is the vaccine authorized for?

A. The vaccine is authorized for people 18 years of age and older. Clinical trials in children are currently underway. Check with your local health department to find out how to make an appointment.

Q. Who should not get the vaccine?

A. You should not get the vaccine if:

  • You have had a severe allergic reaction to any ingredient of this vaccine.
  • You have had a severe allergic reaction to a previous dose of this vaccine.

 You can find more information about COVID-19 vaccines in our Coronavirus health topic center.

Reviewed 12/9/2021

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