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Updated April 28, 2021

Update: On April 23, 2021, the Centers for Disease Control and Prevention (CDC) and U.S. Food and Drug Administration (FDA) released a statement lifting a nationwide pause in the use of the Johnson & Johnson/Janssen COVID-19 vaccine. Federal health officials investigated cases of a rare, serious blood clot reported in some people after they received the J&J vaccine.

After their investigation, the CDC and FDA determined the risk of blood clots from this vaccine is very low and the known benefits in preventing COVID-19 are very high for people 18 and older, outweighing the risks. The vaccine is safe and effective in preventing COVID-19.

On Feb. 27, 2021, the Food and Drug Administration gave emergency use authorization (EUA) for a COVID-19 vaccine developed by Johnson & Johnson/Janssen .

The J&J vaccine became the third COVID-19 vaccine to receive an EUA. Vaccines developed by Pfizer-BioNTech and Moderna received EUAs in December 2020.

Distribution of the J&J vaccine began nationwide in March. Here’s what we know so far about this vaccine.

How Much Johnson & Johnson Vaccine Will There Be?

Johnson & Johnson pledged to have nearly 4 million doses of vaccine ready to ship upon receiving EUA. The company said it would have 20 million doses ready by the end of March and 100 million ready by the end of June.

At UPMC, we have received doses of the J&J vaccine.

Our goal is the same with all vaccine doses we receive: to vaccinate as many people as possible, as quickly as possible, using all of our available supplies. We are following guidance from state health officials on vaccine distribution.

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How Does the Johnson & Johnson Vaccine Work?

The J&J vaccine currently requires a single dose.

The vaccine is known as a viral vector vaccine. According to the Centers for Disease Control and Prevention (CDC), a viral vector vaccine uses a modified version of a different virus to deliver instructions to our cells.

For the J&J vaccine, scientists used an adenovirus — a common virus that usually causes cold or flu symptoms — as the vector. They modified it by adding DNA from SARS-CoV-2, the virus that causes COVID-19. Specifically, they added the gene that can make the coronavirus’ “spike” protein.

When you get this vaccine, the adenovirus enters your cells. Your immune system recognizes that the spike protein doesn’t belong there and produces antibodies against it. The antibodies eventually provide protection against SARS-CoV-2 (and COVID-19).

Is The Johnson & Johnson Vaccine Safe?

While the adenovirus in the J&J vaccine can enter your cells, it has been modified so it can’t cause infection. The vaccine only teaches our body how to create the SARS-CoV-2 spike protein alone, not the virus. Because of that, you cannot get COVID-19 from the vaccine.

You may experience some side effects when you get the vaccine. Common side effects include:

  • Pain and swelling on your arm where you got the shot
  • Fever
  • Chills
  • Headache
  • Fatigue

Most side effects are minor and should go away within a day or two.

Does the Johnson & Johnson vaccine cause blood clots?

In rare cases, some people who received the J&J vaccine developed a rare, serious blood clot. The FDA and CDC temporarily paused the distribution of the vaccine to investigate the blood clot cases.

After investigation, the FDA and CDC determined the vaccine’s benefits in preventing COVID-19 outweigh its risks. These adverse effects are extremely rare.

The J&J vaccine is safe and effective in preventing COVID-19. If you or a loved one receives the J&J vaccine, you DO NOT have to take any medications to prevent blood clots. Call your doctor if you experience side effects related to blood clots, including:

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

“Most people, regardless of the vaccine they receive, should not expect severe side effects like this,” says Donald Yealy, MD, chief medical officer, UPMC. “It’s exceptionally rare.”

Like other treatments and vaccines, the FDA and CDC will continue to monitor the safety of the J&J vaccine.

Is the Johnson & Johnson Vaccine Effective?

Clinical trial data from the J&J vaccine reported it was close to 70% effective in preventing mild to moderate COVID-19. Data showed it was 85% effective in preventing severe disease and 100% effective against hospitalization and death.

Those numbers significantly exceed the FDA requirements to grant an EUA, Dr. Snyder says. The protection against severe illness, hospitalization, and death is especially important.

“If a vaccine protects you 60% to 70% of the time from getting sick, that’s fantastic,” Dr. Snyder says.

“The [Johnson & Johnson, Pfizer, and Moderna vaccines] are all spectacular at keeping you from having to be hospitalized, or in the ICU, or ventilated, or dying. If the job we want a vaccine to do is keep you generally well, they’re all spectacular.”

So far, all three vaccines appear to do a good job to prevent a person from being contagious but without symptoms. More data will be needed on that front, Dr. Snyder says. It is important to continue to wear a mask and maintain distancing.

How Does the Johnson & Johnson Vaccine Compare to Pfizer and Moderna?

The J&J vaccine has some similarities and differences compared to the COVID-19 vaccines produced by Pfizer and Moderna.

How are the COVID-19 vaccines similar?

Reported data show the J&J, Moderna, and Pfizer vaccines are all safe and highly effective in preventing symptomatic COVID-19. They are especially effective against severe COVID-19, hospitalization, and death.

All three vaccines work in similar ways. They all teach our bodies how to make antibodies against SARS-CoV-2 without using a live virus. But the teaching methods are different.

How are the COVID-19 vaccines different?

Differences between the COVID-19 vaccines include:

  • Type of vaccine: Johnson & Johnson’s vaccine is a viral vector vaccine. It uses a modified substitute virus to deliver instructions to your immune system about how to fight SARS-CoV-2. The Pfizer and Moderna vaccines both are mRNA vaccines. They use synthetic messenger RNA to deliver the instructions to your immune system.
  • Number of doses: The J&J vaccine currently requires one dose. The Pfizer and Moderna vaccines both require two doses, delivered weeks apart.
  • Effectiveness: The Moderna and Pfizer vaccines both are at least 94% effective in preventing symptomatic COVID-19, compared to close to 70% for J&J. All three are extremely effective in preventing hospitalization, the need for a ventilator (breathing assistance machine), and death. They will all reduce contagiousness.
  • Storage ability: The J&J vaccine does not require the deep-cold storage of the Moderna and Pfizer vaccines. This should make it easier to ship, store, and potentially distribute. “The more vaccine we can get out, the better,” Dr. Snyder says. “That’s a wonderful thing.”

While the J&J vaccine received the EUA, that does not mean it has full FDA approval. Scientists will continue to monitor the safety and efficacy of the vaccine as distribution continues.

For more information about the COVID-19 vaccine, visit

Centers for Disease Control and Prevention, What to Expect After Getting a COVID-19 Vaccine. Link

Centers for Disease Control and Prevention, Understanding How Viral Vector COVID-19 Vaccines Work. Link

New York Times, F.D.A. Clears Johnson & Johnson's Shot, the Third Vaccine for U.S. Link

New York Times, How the Johnson & Johnson Vaccine Works. Link

About UPMC

Headquartered in Pittsburgh, UPMC is a world-renowned health care provider and insurer. We operate 40 hospitals and 800 doctors’ offices and outpatient centers, with locations throughout Pennsylvania, Maryland, New York, West Virginia, and internationally. We employ 4,900 physicians, and we are leaders in clinical care, groundbreaking research, and treatment breakthroughs. U.S. News & World Report consistently ranks UPMC Presbyterian Shadyside as one of the nation’s best hospitals in many specialties and ranks UPMC Children’s Hospital of Pittsburgh on its Honor Roll of America’s Best Children’s Hospitals. We are dedicated to providing Life Changing Medicine to our communities.