The Food and Drug Administration on Saturday issued 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.

What they're saying: “The authorization of this vaccine expands the availability of vaccines, the best medical prevention method for COVID-19, to help us in the fight against this pandemic,” acting FDA commissioner Janet Woodcock said in a news release.

• “The FDA, through our open and transparent scientific review process, has now authorized three COVID-19 vaccines with the urgency called for during this pandemic, using the agency’s rigorous standards for safety, effectiveness and manufacturing quality needed to support emergency use authorization," Woodcock added.

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