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What to know about the Johnson & Johnson vaccine

A healthcare worker swabs a woman's shoulder.

The Johnson & Johnson (J&J) COVID-19 vaccine was OK'd for emergency use by the U.S. Food and Drug Administration (FDA) on Feb. 27. 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 Moderna vaccines.)

Q. How does the vaccine work?

A. The J&J vaccine uses a viral vector to create immunity. That means it uses another, harmless virus as the vehicle to introduce a piece of DNA to your immune system. That delivery virus is a type that causes the common cold. But it has been modified so that it cannot reproduce in your body or make you sick. It just delivers the DNA package to some of your cells.

Once there, that DNA instructs your cells to make the distinctive but harmless spike protein that appears on the surface of the coronavirus. The cells then display the spike protein on their surface. These proteins trigger an immune reaction. And your body creates antibodies. These protect you from getting sick if you get infected with 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. And it can't change your own DNA in any way.

Q. How many shots are given?

A. This vaccine requires only one shot. So it may be easier for some people to get than vaccines that require two shots, given several weeks apart.

Q. How long after getting your shot 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.

Q. How effective was the vaccine in clinical trials?

A. In international trials, it was about 66% effective at preventing moderate to severe COVID-19. In the U.S. trials, that number rose to 72%. It was 85% effective against severe disease. And no one in the vaccine group died from COVID-19 during the trial.

Those are very good numbers. FDA's benchmark was an efficacy rate of 50%.

It is not yet clear how long the vaccine will provide protection or whether it prevents someone from spreading the virus. But the outlook is good.

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.

No safety concerns were found for any specific groups. Overall, serious adverse events were rare, and 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.
  • Headache.
  • Tiredness.
  • Muscle aches.
  • Nausea.

These were mostly mild to moderate and lasted one or two days.

Q. Why was the vaccine paused?

A. CDC and FDA briefly paused use of the vaccine to investigate reports of an extremely rare type of blood clot in six people who received the J&J vaccine. After finding a total of 15 cases, they determined that the benefits of the vaccine still outweigh the risks and lifted the pause. 

If you receive the J&J vaccine, it's a good idea to watch for these possible symptoms of a blood clot for three weeks after your vaccine:

  • Severe or persistent headache or blurred vision.
  • Shortness of breath.
  • Chest pain.
  • Leg swelling.
  • Persistent abdominal pain.
  • Easy bruising or tiny blood spots under the skin beyond the injection site.

If you have any of these signs or symptoms, get medical help right away.

Q. Who is the vaccine authorized for?

A. The vaccine is authorized for people 18 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 can find more information about COVID-19 vaccines in our Coronavirus health topic center.

Reviewed 9/15/2021

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