The vaccine was 72 percent effective against moderate and severe disease in the US, the company said.

It's a striking difference from vaccines from Pfizer/BioNTech and Moderna, and it may give pause to people uncertain about which vaccine to get or when they can get one. The vaccines already on the market in the US are about 95 percent effective overall against symptomatic COVID-19, with perhaps even higher efficacy against severe cases.

But experts say the Johnson & Johnson vaccine will still be useful against the pandemic in the United States and around the world.

"Among all participants from different geographies and including those infected with an emerging viral variant, Janssen's COVID-19 vaccine candidate was 66 percent effective overall at preventing the combined endpoints of moderate and severe COVID-19, 28 days after vaccination.

The onset of protection was observed as early as day 14," the company said in a statement.

Janssen is Johnson & Johnson's vaccine arm.

"The level of protection against moderate and severe COVID-19 infection was 72 percent in the US, 66 percent in Latin America and 57 percent in South Africa, 28 days post-vaccination."

"We're 85 percent effective at preventing severe disease, which we define as the disease that makes you feel particularly sick at home, or may go to the hospital, or worse," Dr. Mathai Mammen, global head of research and development at Johnson & Johnson, said.

"And we are right now completely protective, it would appear 100 percent protective, against the disease that actually does make you go to the hospital, we're 100% protective against death."

The company said the results held up across all age groups and people of various ethnicities.

Mammen said the company was working to file for emergency use authorization from the US Food and Drug Administration "inside of a week." It will be the third company to seek EUA from the FDA for a coronavirus vaccine. Vaccines made by Moderna and by Pfizer/BioNTech were authorized in December and are now being given to millions of Americans.

Dr. Paul Offit, a vaccine expert at the University of Pennsylvania and a member of the FDA's Vaccines and Related Biological Products Advisory Committee, said that under normal circumstances, there might not be much of a market for a vaccine that is significantly less effective than two others already on the market. But he added that these are not normal times, with a pandemic raging and a shortage of vaccines.

Pfizer and Moderna both use genetic technology called messenger RNA, or mRNA technology. Johnson & Johnson uses a weakened common cold virus, known as adenovirus, to carry genetic instructions into the body to prompt an immune response.

"In a better world, we would have abundant quantities of this messenger RNA vaccine, Pfizer and Moderna, and that would be damning for a vaccine that's clearly less effective. But that said, we have limited quantities of mRNA vaccine," Offit said.

Unlike the Pfizer and Moderna vaccines, Johnson & Johnson vaccine does not have to be stored in freezers. It can be stored for three months at refrigerator temperatures of 36 degrees to 46 degrees Fahrenheit.

Janssen is already testing a two-dose regimen of the vaccine and that could increase the efficacy.

The US federal government has contracted to buy 100 million doses of the vaccine and the company has been manufacturing doses for months in anticipation that it would work and would win EUA from the FDA.

A fourth vaccine is also jostling to join the mix in the US market. Maryland-based Novavax released preliminary data on Thursday showing its candidate vaccine had an efficacy of 89 percent in trials in Britain. And AstraZeneca is finishing up Phase 3 clinical trials of its vaccine, as well.

The Janssen vaccine is made a little differently from the Pfizer and Moderna vaccines, which use raw genetic material called messenger RNA. — Courtesy CNN