The Food and Drug Administration has indicated it would authorize a vaccine that's safe and at least 50% effective. The flu vaccine, by comparison, generally reduces people's risk of getting influenza by 40% to 60% compared with people who aren't inoculated, according to the Centers for Disease Control and Prevention.
The level of protection varied by region, J&J said, with the vaccine demonstrating 66% effectiveness overall, 72% in the United States, 66% in Latin America and 57% in South Africa after four weeks.
J&J said the vaccine was 85% effective in preventing severe disease four weeks after vaccination in all adults. The vaccine offered complete protection against Covid-related hospitalizations four weeks after vaccination, the company said.
"We're proud to have reached this critical milestone and our commitment to address this global health crisis continues with urgency for everyone, everywhere," J&J CEO Alex Gorsky said in a statement.
The highly anticipated results were based on 468 confirmed Covid-19 infections among the phase three trial's more than 43,000 volunteers, according to J&J. The company said the trial includes those infected with B.1.351, the new, highly contagious strain found in South Africa.
Effectiveness against severe disease increased over time with no cases in vaccinated participants reported after day 49, according to the company. Protection was consistent across race and age, including those over age 60.
Dr. Anthony Fauci, the nation's leading infectious disease expert, said the most important finding of the data is the vaccine may keep people out of the hospital and prevent them from getting severe illness.
"The most important thing, more important than whether you prevent someone from getting aches and a sore throat, is preventing people" from getting severe disease, the director of the National Institute of Allergy and Infectious Diseases said on a call with reporters. "That will alleviate so much of the stress and human suffering and death in this epidemic."
J&J said the vaccine was well tolerated, with no significant safety concerns related to the vaccine reported, including anaphylaxis.
Dr. Scott Gottlieb, a former FDA commissioner, called the J&J data "fantastic," saying in a tweet that it showed the vaccine produced "sustained (and increasing!) immune protection over time."
"This one shot vaccine was highly effective at preventing severe disease, even with new variants," he said. "The milieu of disease now is more complex; even in U.S. — trials done today are running into more mutated cases. Make no mistake: this is an important and wonderful development."
U.S. officials and Wall Street analysts are eagerly anticipating the authorization of J&J's vaccine, which could happen as early as next month. Public health officials and infectious disease experts say world leaders will need an array of drugs and vaccines to defeat the virus, which has killed more than 400,000 Americans in about a year.
The new data also comes as U.S. health officials, including Fauci, are concerned that vaccines currently on the market may not be as effective in guarding against new, more contagious strains of the coronavirus. Moderna said Monday it is working on a booster shot to guard against the strain found in South Africa. Pfizer and Novavax also said they will test modified vaccines.
Dr. Paul Offit, director of the Vaccine Education Center at Children's Hospital of Philadelphia, said the new strains are not a problem yet, but it's important for drugmakers to prepare for the possibility that the virus could mutate enough to evade the protection of the current vaccines.
If J&J's vaccine is authorized by the FDA, it would be the third approved for emergency use in the U.S. behind vaccines developed by Pfizer-BioNTech and Moderna. Pfizer's vaccine was authorized by the FDA on Dec. 11, and Moderna's was authorized a week later.
The Pfizer and Moderna vaccines require two doses given about three to four weeks apart. J&J's requires only one dose, easing logistics for health care providers.
Pfizer's vaccine was found to be 95% effective against preventing Covid-19, while Moderna's was found to be about 94% effective. Infectious disease experts, including the University of Toronto's Isaac Bogoch, pointed out that J&J's numbers can't be used as a direct comparison to the other two vaccines because it's a single dose and the company's trial was conducted when there were more infections and new, more contagious variants.
Additionally, J&J has said it plans to ship the vaccine at 36 to 46 degrees Fahrenheit. Pfizer's vaccine needs to be stored in ultra cold freezers that keep it between negative 112 and negative 76 degrees Fahrenheit. Moderna's vaccine needs to be shipped at between negative 13 and 5 degrees Fahrenheit.
The Department of Health and Human Services announced in August that it reached a deal with Janssen, J&J's pharmaceutical subsidiary, worth approximately $1 billion for 100 million doses of its vaccine. The deal gives the federal government the option to order an additional 200 million doses, according to the announcement.
Disclosure: Scott Gottlieb is a CNBC contributor and is a member of the boards of Pfizer, genetic testing start-up Tempus, health-care tech company Aetion Inc. and biotech company Illumina. He also serves as co-chair of Norwegian Cruise Line Holdings' and Royal Caribbean's "Healthy Sail Panel."