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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 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 is eligible six months after their initial vaccine series. The booster does not have to be the Pfizer vaccine. You can stick with Pfizer if you want, or you can choose one of the other available vaccines.

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%.  

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 5 through 15 can get the vaccine under its emergency use authorization. Clinical trials in children under 5 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 11/24/2021

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