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Answering Your Questions About the Johnson & Johnson COVID-19 Vaccine

In February and March, we answered several of your COVID-19 vaccine questions. This month, we’re back with more answers about Johnson & Johnson’s Janssen vaccine (J&J), the third vaccine to be issued emergency use authorization (EUA) by the FDA. If you have specific questions that aren’t answered below, please contact the Fund at

What are the key details of the J&J vaccine?

The EUA allows the J&J vaccine to be distributed in the U.S. to people age 18 and older, the same as Moderna. By comparison, the Pfizer vaccine is approved for people age 16 and older.

  • This vaccine is a single dose and administered as a shot in the upper arm.
  • It does not contain eggs, preservatives or latex.
  • It is a viral vector vaccine.

What is a viral vector vaccine?

A viral vector vaccine uses a harmless version of a different virus to deliver information to the body that helps it protect you. Viral vector vaccines were first created by scientists in the 1970s and have been studied to treat cancer and in recent Ebola outbreaks. There have been a number of studies focused on using viral vector vaccines against other infectious diseases like Zika and the flu.

How does the J&J vaccine work?

The viral vector in the J&J vaccine is replication-incompetent – which means it can’t replicate in your body. The adenovirus vector uses a piece of the inactivated common cold virus to deliver a piece of the spike protein to your immune system so that you start to make antibodies. If you are exposed to the real virus (SARS-CoV-2) later, your body will recognize it and know how to fight it.

The vaccine does not contain the virus that causes COVID-19 and can’t give you COVID-19. It also can’t make you sick from the common cold virus that is used as a vector. It can’t change your DNA in any way.

What side effects should I expect from the J&J vaccine?

Like the Pfizer and Moderna vaccines, common side effects can include pain, redness and swelling in the arm where you got the shot, along with tiredness, headache, muscle pain, chills, fever and nausea throughout the rest of the body.

Side effects usually start within a day or two of getting the vaccine and are most common within seven days of getting vaccinated. Side effects are more common in people younger than age 60.

How well does the J&J vaccine work?

The J&J vaccine becomes more effective over time, reaching peak effectiveness 28 days after vaccination. It’s 77 percent effective at preventing severe/critical COVID-19 infection 14 days after vaccination and 85 percent effective at preventing severe/critical COVID-19 infection 28 days after vaccination. It is 100 percent effective in preventing hospitalizations and deaths related to COVID-19.

Is one vaccine better than the other?

One vaccine is not better than another. If you are offered the J&J vaccine, you should take it. If you are offered the Pfizer vaccine, you should take it. If you are offered the Moderna vaccine, you should take it. The best vaccine to get is the vaccine that is available to you.

What changes after I’m fully vaccinated?

Some preventive actions are no longer needed once you are fully vaccinated (two weeks after your second dose of Pfizer or Moderna or two weeks after your single J&J dose). Others are still recommended depending on the location, the number of people you are spending time with and their vaccination status. For more detailed information and recommendations for after you’re vaccinated, check out the Fund’s Guidance for People Vaccinated Against COVID-19.

For more information about COVID-19 vaccines in general, check out the Fund’s latest fact sheet, COVID-19 Vaccines: Key Facts & Benefits.

[Emily Smith is the LHSFNA’s Health Promotion Manager.]

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