Federal authorities have authorized the emergency distribution of Johnson & Johnson's Janssen COVID-19 vaccine, which is designed to be given as a single dose. 

A recent CNN report explained that this means no follow-up visits and none of the red tape needed to make sure people return for second shots, in addition to none of the worry about making sure a second dose is available at the right time.

This is one of the major differences between Johnson & Johnson's vaccine and its Pfizer and Moderna counterparts, which are both designed to be given in a two-dose series -- the Pfizer vaccine three weeks apart and Moderna's four weeks apart.

There's been some discussion as to whether it might be acceptable to administer these vaccines as a single dose, or to extend the time between doses so more people can get their first vaccine, and thus get at least some protection.

However, the FDA authorization says two doses are acceptable at this time. A number of vaccine experts, including White House chief medical adviser Dr. Anthony Fauci, agree, fearing that by administering only one dose of Pfizer and Moderna's vaccines might leave people partialy protected.

In contrast, Johnson & Johnson's vaccine was tested and proven to protect people with a single dose, although studies are underway to see if two doses might provide even more protection.

But, how does Johnson & Johnson's vaccine stack up against Pfizer's and Moderna's in terms of efficacy?

According to CNN, those vaccines had a startling efficacy rate in clinical trials of 94% to 95%.

The news outlet states that based on statistics in real-world studies of Pfizer's vaccine in Israel, efficacy holds up. The risk of symptomatic COVID-19 -- meaning people who got infected with the coronavirus and felt sick -- decreased by 94% among people who were given two doses of the vaccine.

In comparison, the overall global efficacy of Janssen's vaccine was 66% against moderate to severe illness.

That said, it was 85% effective against severe disease and, in trials anyway, 100% effective at preventing death, as no one who got the vaccine died from COVID-19.

CNN notes that the Johnson & Johnson vaccine was administered in different populations and at a different time. It was tested in 44,000 people in the US, South Africa and Latin America, and most of the testing was months later in the pandemic than the Pfizer and Moderna vaccines, which started testing in the spring and summer.

This is somewhat different from how testing of Pfizer's vaccine proceeded. The vaccine was tested in 43,000 people in the United States, Germany, Turkey, South Africa, Brazil and Argentina. And, Moderna's was tested in 30,000 people, all of them in the US.

The Johnson & Johnson vaccine was tested after some of the troubling new coronavirus variants had started to circulate, CNN explains, including one first seen in South Africa, called B.1.351, that appears to weaken the body's recognition of the virus -- including after vaccination. The Johnson & Johnson vaccine's efficacy was just 57% in South Africa, where B.1.351 is now the dominant variant, compared to 72% in the US, where it is far less common.

Health experts seem to agree that all three of the vaccines provide very good protection by the most important measure, which is whether they keep people from getting seriously ill.

But the differing efficacies raise the possibility that some will perceive the Johnson & Johnson vaccine as second class, Sarah Christopher, the policy advocacy director at the National Women's Health Network, told an FDA advisory committee meeting Friday.

There are feelings "that there are first and second class vaccines, with the latter relegated to low income, rural, or otherwise marginalized communities that has the potential to exacerbate existing mistrust," she said. "Public health authorities must address these perceptions head on."

Johnson & Johnson's vaccine begins to protect patients against moderate to severe disease starts about two weeks after people get vaccinated. Four weeks after the shot, data from the clinical trial showed there were no hospitalizations or deaths.

Recent studies show good level of protection with the first dose of the Moderna and Pfizer vaccines, but people don't get full protection until about two weeks after the second dose -- so five to six weeks after the first dose, CNN reports.

The news outlet goes on to say that Moderna and Pfizer vaccines use a brand-new technology called messenger RNA, or mRNA. They deliver genetic material directly into cells via fatty particles. That genetic code is taken up by cells in the arm muscle, which then follow the genetic instructions to make tiny pieces that look like a part of the coronavirus.

Those small proteins stimulate an immune response, generating antibodies and immune cells that "remember" what they look like and that will be ready to respond quickly in case of a new attack.

According to CNN, the Johnson & Johnson vaccine uses viral vector technology. A common cold virus called adenovirus 26 is genetically engineered so that it can infect cells, but it won't replicate there. It cannot spread in the body, and won't give people a cold, the news outlet explains. Similar to the Pfizer and Moderna vaccines, it delivers genetic instructions.

Instead of being carried in little lipid balls, the genetic instructions are injected by the weakened virus into arm cells, and they make the pieces that look like part of the coronavirus spike protein -- the knob-shaped structure that the virus uses to connect to cells, CNN reports.

Those delicate little balls of fat used to carry the Moderna and Pfizer vaccines require careful handling. Pfizer's vaccine needs to be stored and shipped at between -80ºC to -60ºC (-112ºF to -76ºF) -- something that caused a lot of trouble for states at first, which had to scramble to get dry ice and special freezers.

As of March, the FDA has eased up on those requirements a bit, but the vaccine can still only be held in the refrigerator for five days and must be used within six hours of being thawed and diluted.

The Pfizer vaccine must be diluted before it's used, and it cannot be shaken, but must be carefully inverted exactly 10 times to mix it. The Moderna vaccine is a little less complicated, but must also be frozen and carefully handled.

Moderna's vaccine can be kept at about -20ºC, or about the temperature of a home freezer, CNN reports.

Johnson & Johnson's vaccine is much easier to store. It can be kept at simple refrigerator temperatures for up to three months, making it far easier to store and ship.

None of the three vaccines have additives that can sometimes cause strong reactions, such as antibiotics, preservatives or adjuvants, which are compounds used to boost the immune response that can add to the kick of any vaccine.

This means that risk of allergic reaction is low, especially life-threatening anaphylaxis. The CDC reports only a few cases of anaphylaxis in people who have been given the Moderna or Pfizer vaccines.

Only one case of anaphylaxis has been reported in the 44,000 people who have tested the Johnson & Johnson vaccine.

In addition to this, all three vaccines target a specific part of the spike protein called the receptor binding domain. As the name implies, it is the part the virus uses to grapple cells. Mutations to this particular region might weaken the efficacy of all three vaccines.

Fortunately, all three appear to stimulate an overwhelming immune response and so far, it appears to be enough to continue at least partially protecting people from the most concerning variants.