Johnson & Johnson's (JNJ) single-dose COVID-19 vaccine was given an emergency use authorization (EAU) Saturday evening by U.S. Food and Drug Administration (FDA), making it the third vaccine available in the U.S.

The Vaccines and Related Biologics Products Advisory Committee (VRBPAC) voted unanimously Friday to recommend use of the vaccine on adults aged 18 and older, based on all the data submitted by the company, and the vaccine was endorsed by FDA staff earlier in the week.

J&J's vaccine, produced by its pharmaceutical arm Janssen, was 66% efficacious globally, and 72% in the U.S., according to documents submitted to the FDA. It puts the vaccine within the acceptable range for authorization.

The two currently authorized vaccines — by Moderna (MRNA) and Pfizer (PFE)/BioNTech (BNTX) — have more than 90% efficacy, but experts have said an apples-to-apples comparison with J&J's vaccine is hard to do since the two earlier vaccines were tested in an environment that wasn't as impacted by concerning variants.

Dr. Cody Meissner, a member of the vaccine advisory committee and pediatric infectious disease expert at Tufts Children's Hospital, did note all vaccines are showing similar protection from severe cases.

"All the vaccines seemed to be equally effective at preventing very severe disease, intensive care need, and deaths," Meissner said.

He also wondered whether or not the FDA, as a result, should change the basis by which vaccines should be judged in the future, moving away from the overall efficacy rate.

Unlike the earlier meetings, a significant portion of the early part of Friday's meeting was dedicated to defining and understanding variants, as well as understanding the purpose of genomic sequencing to identify variants — something the U.S. has not done as well as other countries. Experts expect strains will become dominant in the U.S. this year, replacing the original strain from Wuhan, China.

The mutations are a concern for experts as it could make the virus a "moving target" for the vaccines, according to Adam MacNeil, staff epidemiologist for the U.S. Centers for Disease Control and Prevention (CDC).

MacNeil noted a pending question is whether or not we will need "annual updates, like influenza (flu), will we need annual updates every 5 years, or will we have broader protection?"

J&J's vaccine held up against the concerning variant originating in South Africa (B.1.351), but there was no evidence of the variants originating in the U.K. (B.1.1.7) and Brazil (P.1) in the trial group, so efficacy against those strains is unknown. However, the B.1.351 variant is the most concerning as it has shown some resistance to vaccines.

Both Novavax (NVAX) and J&J vaccines show clinical trial evidence of reduced effectiveness, while Pfizer (PFE)/BioNTech (BNTX) and Moderna (MRNA) showed lab proof of reduced efficacy. The latter two just announced they've created booster shots targeting variants that will be tested in clinical studies. Moderna created a B.1.351-specific booster, but it is also testing the potential of a half dose of the current vaccine as a booster.

Since the vaccine approval process has been expedited in the U.S., ongoing observations of trial participants beyond the end of Phase 3 can be disrupted, as it has been in previous company's trials.

Active and passive safety surveillance, as well as observational studies to continue following up on trial participants to ensure any long-term effects of the vaccine, are typically done for at least six months but can also be up to two years, according to the experts Friday.

Data from Johnson & Johnson showed a total of 16 deaths in clinical trials — none related to the vaccine. In the vaccinated group, none of the deaths were from COVID-19.

If authorized, J&J's adenovirus vector vaccine would be a game-changer as it only requires one shot and can be kept at more normal refrigerator temperatures. The mRNA vaccines, by comparison, require freezing temperatures and two shots.

But the single-dose designation could change, which was a point of concern among experts Friday. J&J is currently studying the efficacy of two doses of its vaccine, with results expected in May. Dr. Paul Offit, a member of the committee and a vaccine expert at the Children's Hospital of Philadelphia, noted it will be challenging to explain to the general public that they might have to get a second dose if the results show as much.

Initially, concerns over Pfizer's ultra-cold storage needs were viewed as a hurdle to capturing a broader share of the COVID-19 vaccine market globally, but the company has completed studies on its vaccine stability at normal freezer temperatures, putting it on par with Moderna.

An authorization could come as soon as Saturday. Moderna and Pfizer both received EUAs the day after the VRBPAC meeting.

Once a EUA is received, J&J reports it has 4 million doses ready to ship, with 20 million expected by the end of March. The original contract called for 12 million by the end of February. But the company says it will fulfill its target of 100 million doses by June. Meanwhile, the federal government is looking at ways to help J&J accelerate its vaccine production domestically.

More from Anjalee:

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• What you need to know about at-home, over-the-counter COVID-19 tests

• Biden COVID-19 adviser: Vaccine news is great, but we still have a ways to go

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