The Food and Drugs Administration on Saturdayissued an emergency use authorization for Johnson & Johnson's one-shot coronavirus vaccine.

Why it matters: The authorization of a third coronavirus vaccine in the U.S. will help speed up the vaccine rollout across the country, especially since the J&J shot only requires one dose as opposed to Moderna and Pfizer-BioNTech's two-shot vaccines.

• Unlike Moderna and Pfizer-BioNTech's shots, the J&J vaccine can also be stored at refrigerator temperatures for three months, making it easier to transport.
• White House coronavirus coordinator Jeff Zients said on Wednesday that J&J will have 3 million to 4 million doses ready for distribution.

By the numbers: The vaccine was found to be 66.9% effective against moderate to severe/critical COVID-19 cases 14 days after vaccination, and 66.1% effective after 28 days. Against severe/critical cases, the vaccine was 76.6% effective after 14 days and 85.4% effective after 28 days.

• A large clinical trial showed no COVID-19 hospitalizations or deaths 28 days after patients received the vaccine.
• "The analysis supported a favorable safety profile with no specific safety concerns identified that would preclude issuance of an EUA," FDA staff wrote in a briefing document released Wednesday.

Be smart: Infectious disease experts have noted that J&J's numbers can't be directly compared to those of Pfizer or Moderna, which were found to be about 94%-95% effective, because J&J's vaccine is a single dose, among other factors, per CNBC.

The big picture: J&J has a deal with the U.S. government to provide 100 million doses by the the end of June.

• More than half of the J&J doses sold thus far are destined for the developing world, with 500 million doses purchased by the global COVAX initiative and 120 million by the African Union.

Go deeper: Fauci urges Americans to take whatever COVID vaccine is available