How does Johnson & Johnson's COVID-19 vaccine stack up against shots from Pfizer and Moderna?

• Johnson & Johnson's COVID-19 vaccine was shown to be 66% effective at preventing mild and moderate infections, and 85% effective at preventing severe disease, in FDA data released Wednesday.
• It doesn't appear to be as effective as Moderna or Pfizer's mRNA vaccines, though it is nearly just as good at preventing hospitalizations and deaths.
• It has the benefits of being cheap and relatively easy to manufacture and distribute. It only requires one shot in the arm, and side effects tend to be milder.

The US Food and Drug Administration on Saturday authorized Johnson & Johnson's coronavirus vaccine.

The shot is 66% effective at preventing moderate to severe COVID-19, according to data released on Wednesday.

At first glance, the vaccine doesn't appear to be as stellar as FDA authorized vaccines from Pfizer and Moderna, but it does have a few perks.

First and foremost: J&J's vaccine is just one shot in the arm, while both Pfizer and Moderna's are administered as two doses, given several weeks apart. Second, it can be refrigerated for three months, making transportation and storage far less of a challenge. Third, it appears to have a milder set of side effects. And finally, it is cheaper and easier to produce.

"Our goal all along has been to create a simple, effective solution for the largest number of people possible, and to have maximum impact to help end the pandemic," Johnson & Johnson Chief Executive Officer Alex Gorsky said in a statement when his company's results were released in late January.

During a call at the National Institutes of Health last month, Dr. Anthony Fauci, the US' top infectious disease expert, also stressed the importance of adding "yet another platform, another candidate into the mix in our global efforts against this extraordinary pandemic."

"This has really important domestic and global public health implications," Fauci said of the J&J vaccine, noting that "it is one shot," that doesn't need to be stored at super-chilled temperatures, and it is relatively inexpensive.

Below are the basics of how these three COVID-19 shots compare, based on what we know so far.

How well the shots prevent illnesses:

Moderna: 94.1% effective at preventing symptomatic infections, 100% effective at preventing severe infections, after two doses.

Pfizer: 95% effective at preventing symptomatic infections, nearly 100% effective at preventing severe infections (one case among more than 18,000 vaccinated individuals in the trials), after two doses.

Johnson & Johnson: 66% effective at preventing moderate to severe infections, 85% effective at preventing severe infections, after a single dose. (There were five confirmed severe cases among more than 19,000 fully vaccinated individuals in the trials, meaning people who'd received the shot at least 28 days prior.)

How well the shots prevented hospitalizations and deaths:

Moderna: Zero hospitalizations or deaths 14 days after the second shot.

Pfizer: Zero hospitalizations or deaths seven days after the second shot.

Johnson & Johnson: Zero hospitalizations or deaths 28 days after the single shot.

How many people tried them out:

Moderna: trials included more than 30,000 volunteers across the US.

Pfizer: trials included more than 40,000 volunteers across six countries: the US, Argentina, Brazil, Turkey, Germany, and South Africa.

Johnson & Johnson: trials included more than 40,000 volunteers across eight countries: the US, Argentina, Brazil, Chile, Colombia, Mexico, Peru, and South Africa.

About half of the volunteers in each company's study got real shots, and half got fake (placebo) jabs, to test out how well the vaccines work, compared to nothing.

How they work:

Moderna: mRNA.

Pfizer: mRNA.

Johnson & Johnson: adenovirus. (It's the same platform as an Ebola vaccine that was approved for use in the European Union last July.)

How you take them:

Moderna: Two shots in the arm, given 28 days apart.

Pfizer: Two shots in the arm, given 21 days apart.

Johnson & Johnson: One shot in the arm.

Most common side effects:

Moderna: arm pain (91.6%), fatigue (68.5%), headache (63.0%), muscle pain (59.6%), joint pain (44.8%), and chills (43.4%).

Pfizer: arm pain (84.1%), fatigue (62.9%), headache (55.1%), muscle pain (38.3%), chills (31.9%), joint pain (23.6%), fever (14.2%).

Johnson & Johnson: arm pain (48.6%), headaches (38.9%), fatigue (38.2%), and muscle aches (33.2%).

How long it takes for protection to kick in:

Moderna: some protection from infection begins 10-14 days after the first shot, with full protection achieved two weeks after the second shot.

Pfizer: some protection from infection begins 10-14 days after the first shot, with full protection achieved one week after the second shot.

Johnson & Johnson: some protection from infection begins as early as 14 days after the shot, with full protection measured 28 days after the jab.

How long hospitals and clinics can store it in the fridge

Moderna: 30 days.

Pfizer: Five days.

Johnson & Johnson: Three months.

"The company can actually produce, in a reasonable period of time, billions of doses," Fauci added, suggesting that though this vaccine may not be as potent as the mRNA vaccines from Pfizer and Moderna, it could become a more widespread tool to help suppress the pandemic.

This story has been updated with new information from FDA briefing documents and with additional details after the FDA authorized J&J's vaccine.