Johnson & Johnson’s COVID-19 vaccine is safe and effective, U.S. regulators said, a key milestone on the path toward giving Americans access to the first such shot to work in a single dose.

The vaccine was 72 per cent effective in a U.S. clinical trial, Food and Drug Administration staff wrote in a document summarizing the company’s trial data, confirming findings J&J released earlier this month. There were no COVID-related deaths in the vaccinated group, the staff wrote. Agency officials prepared the document ahead of a meeting Friday where external advisers will make a non-binding recommendation as to whether the vaccine should be authorized.

The analysis supported a favorable safety profile with no specific safety concerns identified that would preclude issuance of an EUA.

Vaccines from Moderna Inc. and the partnership of Pfizer Inc. and BioNTech SE were authorized by the FDA in December, just before the first major coronavirus variants were seen in the U.S. Since then, mutant versions have become a major concern, and J&J’s vaccine has been found to be less effective in South Africa and Latin America, where two quickly-spreading versions were identified. Another variant that first arose in the U.K., called B.1.1.7, has been seen at even higher levels in the U.S.

If J&J’s shot is quickly authorized after Friday’s advisory panel meeting, as the other two vaccines were, the company will initially be able to provide 4 million doses, according to Richard Nettles, the vice president of U.S. medial affairs for J&J’s Janssen Infectious Diseases and Vaccines unit.

Speaking in a Congressional hearing on Tuesday, Nettles said that J&J will likely produce enough of its one-dose vaccine to inoculate more than 20 million Americans by the end of March.

The New Brunswick, New Jersey-based company’s contract calls for 100 million doses delivered by the end of June.

Unlike the Moderna and Pfizer-BioNTech vaccines that require two doses given three to four weeks apart, J&J’s shot can be stored at refrigerator temperatures for up to three months. J&J’s vaccine is made from a cold virus that doesn’t replicate but helps the body develop an immune response and prepare for a COVID-19 infection.

The most frequent side effects included fever, fatigue, headache, muscle pain and injection-site pain, FDA staff wrote.