Bloomberg Law
Feb. 25, 2021, 10:30 AM

One Dose and Variant-Tested: The J&J Covid-19 Shot Explained

Jacquie Lee
Jacquie Lee

Johnson & Johnson’s Covid-19 shot will be a valuable addition to the arsenal of vaccines that could get the world closer to herd immunity.

Health leaders expect the Food and Drug Administration to green-light the shot, which will become the first single-dose Covid-19 vaccine available in the U.S. It will be a huge asset to boosting vaccine rates by increasing supply and ensuring people can get fully immunized without having to go back for a second shot.

1) How is J&J’s vaccine different from the other shots?

The biggest difference between the J&J shot and the vaccines by Moderna Inc. and Pfizer Inc. is that the latter require two doses. J&J’s vaccine is also based on a different technology.

Moderna and Pfizer’s shots use a messenger RNA platform, which instructs cells in the body to make proteins that fight off or prevent disease. J&J’s shot is based on a cold virus, called an adenovirus, that doesn’t replicate but helps the body develop an immune response against the coronavirus. J&J used the same technology for its vaccine to fight Ebola.

Adenovirus-based vaccines are sturdier than mRNA vaccines, which is why the J&J vaccine can be stored at warmer temperatures than Pfizer or Moderna’s products.

J&J’s vaccine is estimated to remain stable for two years at minus 4 degrees Fahrenheit. It can stay at temperatures of 36–46 degrees for at least three months. Pfizer and Moderna’s vaccines require ultra-cold storage of minus 94 and minus 4 degrees Fahrenheit, respectively. However, Pfizer recently said its product could be stored for two weeks between minus 13 and 5 degrees Fahrenheit.

2) Which vaccine is the most effective?

J&J’s vaccine was 72% effective overall in preventing moderate to severe Covid-19—the disease caused by the SARS-Cov-2 virus—in U.S. trials. The Pfizer and Moderna vaccines are both about 95% effective.

However, the J&J vaccine is 86% effective at preventing hospitalization for Covid-19, according to an FDA briefing document.

Despite the lower efficacy rate compared to Moderna and Pfizer, the J&J vaccine still shields people from the worst of Covid-19 symptoms and death.

3) Will it work against virus mutations?

Clinical trials in South Africa revealed the strain in that area diminished the J&J vaccine’s efficacy rate to 64%, as compared to the 72% efficacy rate against the original strain in the U.S.

There isn’t definitive data on how well the Moderna and Pfizer vaccines work against new strains, which means the single-dose J&J shot will be the first vaccine authorized using robust data on its efficacy against virus variants.

Scientists worry new strains of the SARS-Cov-2 virus will dull the efficacy of existing Covid-19 vaccines. That’s why the FDA wants Moderna and Pfizer to start studies to develop booster shots soon. The studies can be a few hundred people and will take two to three months to run.

Moderna said it already made a booster shot for the South African variant and sent it to the National Institutes of Health to be tested.

In this video, we answer the question on every employees’ mind: can their employers make them take the vaccine?

4) Can I choose which shot I want?

Supply is still so limited for any vaccine that you won’t be able to pick which one you get. That could change this summer when supply is expected to increase, but it’s not a priority for vaccine administrators right now.

The most important thing to do is get a shot when you’re offered one.

—With assistance from Jeannie Baumann

To Learn More:

—From Bloomberg Law:

FDA Creates Booster Shot Clearance Path to Fight Virus Variants

Multiple Covid-19 Vaccine Protections Pose Messaging Challenge

Double-Dose Covid Shots Face History of People Skipping Last Jab

Fauci Curious to See Secret of Covid Shot’s Success Unlocked

Who Should Avoid a Covid-19 Vaccine: Severe Allergies Explained

—From Bloomberg News:

J&J Vaccine Provides Strong Shield Against Severe Covid

J&J Covid Vaccine Found Effective by FDA Before Panel Meets

U.S. Will Have Enough Vaccine for 130 Million by End of March

Second-Shot Crunch Leaves Many Without Complete Immunizations

To contact the reporter on this story: Jacquie Lee in Washington at

To contact the editors responsible for this story: Fawn Johnson at; Alexis Kramer at; Jo-el J. Meyer at