The first one-shot COVID-19 vaccine provides good protection against the illness, Johnson & Johnson reported in a key study released Friday, offering the world a potentially important new tool as it races to stay ahead of the rapidly mutating virus.

The pharmaceutical giant’s preliminary findings suggest the single-dose option may not be as strong as Pfizer’s or Moderna’s two-dose formula, and was markedly weaker against a worrisome mutated version of the virus in South Africa.

But amid a rocky start to vaccinations worldwide, that may be an acceptable trade-off to get more people inoculated faster with an easier-to-handle shot that, unlike rival vaccines that must be kept frozen, can last months in the refrigerator.

“Frankly, simple is beautiful,” said Dr. Matt Hepburn, the U.S. government’s COVID-19 vaccine response leader.

J&J plans to seek emergency use authorization in the U.S. within a week. It expects to supply 100 million doses to the U.S. by June — and a billion doses globally by year’s end — but declined to say how much could be ready if the Food and Drug Administration gives the green light.

Defeating the scourge that has killed more than 2 million people worldwide will require vaccinating billions. The shots being rolled out in different countries so far all require two doses a few weeks apart for full protection. Nearly 23 million Americans have received a first dose of Pfizer or Moderna shots since vaccinations began last month, but fewer than 5 million have gotten their second dose.

Also Friday, regulators cleared a third option, AstraZeneca’s vaccine, for use throughout the European Union. The decision came amid criticism that the 27-nation bloc is not moving fast enough, as well as concern that there’s not enough data to tell how well the vaccine works in older people.

J&J studied its one-dose option in 44,000 people in the U.S., Latin America and South Africa. Interim results found the shot 66% effective overall at preventing moderate to severe COVID-19, and much more protective — 85% — against the most serious symptoms. There were no serious side effects.

“Gambling on one dose was certainly worthwhile,” Dr. Mathai Mammen, global research chief for J&J’s Janssen Pharmaceutical unit, told The Associated Press.

The vaccine worked better in the U.S. — 72% effective against moderate to severe COVID-19 — compared with 66% in Latin America and 57% in South Africa, where a more contagious mutant virus is spreading.