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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. While it is still authorized, experts have recommended a temporary pause in the use of the vaccine while they investigate reports of a rare blood clot in a small number of women who received the vaccine.

In the meantime, here are some important things to know about this vaccine.

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. So it will be important for those who get the vaccine to continue taking other safety precautions.

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 has the vaccine been paused?

A. CDC and FDA are currently investigating reports of an extremely rare type of blood clot in six women who received the J&J vaccine. If you've received the J&J vaccine in the past three weeks, tell your doctor right away if you experience: 

  • Severe headache.
  • Backache.
  • New neurologic symptoms.
  • Severe abdominal pain.
  • Shortness of breath.
  • Leg swelling.
  • Tiny red spots on the skin.
  • New or easy bruising.

The risk of developing this type of blood clot is very low. But while they investigate, CDC and FDA have recommended a temporary pause in use of the vaccine.

Q. Who is the vaccine authorized for?

A. The vaccine is authorized for people 18 and older. Clinical trials in children are currently underway.

Q. Who can get the vaccine?

A. Use of the vaccine is temporarily paused. Otherwise, states set their own rules for distributing vaccines. Check with your local health department to find out whether the vaccine is available to you yet.

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.

If you are pregnant or breastfeeding, talk with your doctor about whether to be vaccinated.

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

Reviewed 4/16/2021

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