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What to know about the Pfizer vaccine

A gloved hand draws vaccine into a syringe from a vial in front of an image of the coronavirus.

Pfizer's COVID-19 vaccine, Comirnaty, was the first to be fully approved by the U.S. Food and Drug Administration (FDA). Millions of people have received it safely so far.

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

Q. How does the vaccine work?

A. The Pfizer 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 a 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 three weeks apart. Some people may need a third booster shot as well, including people with weakened immune systems, older adults, those living in long-term care facilities, people with underlying medical conditions, and people at high risk because of where they work or live. Talk to your provider about what's right for you.

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 91% effective in preventing COVID-19 in clinical trials. That's very good. FDA's benchmark was an efficacy rate of 50%.

The vaccine appears to provide long-term protection, but it's possible a booster shot may be needed at some point.

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 common side effects among those who got the vaccine were similar to other vaccines, such as:

  • Fatigue.
  • Headache.
  • Muscle pain.
  • Chills.
  • Injection site pain.

These reactions were more likely to be reported after people got the second of the two vaccine doses.

Q. Who is the vaccine authorized for?

A. The vaccine is fully approved for people 16 and older. Kids ages 12 through 15 can get the vaccine under its emergency use authorization. And some people with compromised immune systems can get a third booster shot. Clinical trials in children under 12 are currently underway.

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 much more information about COVID-19 in our Coronavirus health topic center.

Reviewed 10/18/2021

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