The U.S. Food and Drug Administration (FDA) approved Johnson & Johnson (J&J)'s single-dose COVID-19 vaccine for emergency use on Saturday, the third vaccine to be greenlighted in the country after those from Pfizer and Moderna.

In J&J's 44,000-person global trial, the vaccine was found to be 66 percent effective in preventing moderate-to-severe COVID-19 infection four weeks after inoculation, and 100 percent effective in preventing hospitalization and death due to the virus.

"This is an emergency use authorization to really mitigate hopefully the devastating effects of the current pandemic," Marion Gruber, the FDA's director of the office of vaccines research and review, said during an advisory panel meeting on Friday.

Easier to distribute

J&J's vaccine is expected to be used widely around the globe because it can be shipped and stored at normal refrigerator temperatures, making distribution easier than for the Pfizer-BioNTech and Moderna vaccines, which have to be shipped frozen.

"It potentially could play a very substantial role if we have enough doses because it's only a single-dose vaccine and that will make it attractive to people who are difficult to reach," said Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center in Nashville. "It's one and done."

Schaffner said the fact that the J&J vaccine can be stored at normal refrigeration temperatures will make it easier to distribute. "We can bring the vaccine to the people," he said.

U.S. purchases 100 million doses

The U.S. government has purchased 100 million doses of the J&J vaccine, which are expected to be delivered by June. 

It plans to distribute about 3 million to 4 million this week. That would be on top of the around 16 million doses of the Pfizer-BioNTech and Moderna vaccines it already planned to ship across the country.

J&J plans to provide a total of 20 million doses by the end of March, which along with the more than 220 million total doses expected from Pfizer and Moderna would be enough to fully vaccinate 130 million people in the U.S.

So far, the U.S. has distributed more than 90 million doses of COVID-19 vaccines, some of which have been used for second shots. About 14 percent of Americans have received at least one dose, according to the U.S. government data. 

COVID-19 has claimed more than half a million lives in the U.S. and states are clamoring for more doses to stem cases, hospitalizations and deaths.

Public health officials have warned about a rise in the prevalence of more contagious variants of the virus, adding to the urgency to get millions more people vaccinated as quickly as possible. Meanwhile, they said, mask wearing and other measures to curb the virus spread should remain in place as recent declines in COVID-19 cases appear to be leveling off.

The J&J vaccine is also under review by the European Union, where deliveries are expected starting in April and would build on the region's thin supplies of the Pfizer-BioNTech, Moderna and AstraZeneca shots.

Efficacy against virus variant

In South Africa, regulators were awaiting the FDA decision as their government looks to deploy more J&J vaccine against a variant of the virus called B.1.351 that is able to evade some vaccine protection.

J&J's vaccine is being rolled out there prior to official authorization for about 500,000 healthcare workers as it tries to stem infections from the variant, which has swept across the country and spread globally, including to the United States.

The vaccine is one of the few that has been tested in clinical trials against the variant and had a 64 percent efficacy rate at preventing moderate-to-severe disease in South Africa.

J&J said on Friday that the company was developing a second-generation vaccine that would target the concerning variant and will be ready to start Phase I trials by this summer.

J&J's vaccine uses a common cold virus, known as adenovirus type 26, to introduce coronavirus proteins into cells in the body and trigger an immune response.

Both the Pfizer and Moderna vaccines, which are based on new messenger RNA technology, showed higher efficacy rates in their pivotal trials when tested using two doses versus J&J's single-shot vaccine. But experts cautioned against close comparisons because the trials had different goals and J&J's was conducted while the more contagious new variants of the virus were circulating.

J&J is testing a two-dose version of its vaccine, with results expected this summer.