By Peter Loftus

Johnson & Johnson reported that its single-dose Covid-19 vaccine protected against Covid-19 in a late-stage trial, while being generally safe and well tolerated. Here's what we know and don't know:

How effective is J&J's vaccine?

It appears to work well. The vaccine was 66% effective in a late-stage study with about 44,000 volunteers ages 18 years and older, according to the company. That indicates the shot protects adults from moderate to severe Covid-19, the disease caused by the coronavirus. When focusing on preventing severe disease specifically, the vaccine was even more effective, posting an 85% rate. By comparison, an annual flu shot is considered to work well if it is 60% effective.

How does it compare to the effectiveness of Pfizer and Moderna shots?

The vaccine from Pfizer Inc. and partner BioNTech SE was 95% effective at preventing people from getting sick with Covid-19 in its late-stage study, while the Moderna vaccine was 94.1% effective. J&J's shot didn't work as well in its trial. It can be tricky to compare across trials, however. And the timing of the testing might have played a role, as J&J's took place while new coronavirus variants emerged. Vaccines don't appear to work as well against the variant first identified in South Africa, in particular. J&J's vaccine was 57% effective in South Africa compared with 72% in the U.S. during the late-stage trial. Several vaccines have performed above what health experts say is needed to protect many people and provide the community immunity needed to move to a post-pandemic life.

When might the vaccine become available?

J&J says it expects in early February to ask the U.S. Food and Drug Administration to authorize the use of its vaccine. If the FDA takes the same amount of time as it did to review and authorize the first two Covid-19 vaccines -- about three weeks -- it could make a decision later in February or by early March. J&J has been making doses while testing its shot, so it could be ready to ship supplies quickly after getting a go-ahead from regulators.

What are the side effects of J&J's vaccine?

The most common symptoms after vaccination were fatigue, headache, muscle ache and injection-site pain in an early-stage study. In the late-stage study, J&J said a small percentage of subjects had fever, and there were no severe allergic reactions.

Which vaccine should I get?

The first one you can, health authorities say. It is possible that individuals won't have much of a choice because of limited supplies and the vaccination site might only offer one of the shots. If you do have a choice, there are some key differences that could guide your decision. J&J's vaccine, while appearing less effective in studies, is given as a single dose, which may be a more convenient option than the two doses required for both of the Pfizer and Moderna vaccines, given three or four weeks apart. Only the Pfizer vaccine is authorized for adolescents 16 and 17 years old, while Moderna's is cleared for those 18 and older. J&J's also is likely to be cleared for adults because only people 18 years and older were in the company's large clinical trial.

How was J&J's vaccine tested?

Starting in September, J&J enrolled more than 44,000 adults in the U.S. and several other countries including Brazil and South Africa in a clinical trial. Subjects received a single dose of either the vaccine or a placebo. Researchers counted how many people subsequently contracted moderate to severe Covid-19 starting 14 days after vaccination, until a certain number of people fell ill. Researchers then examined whether there were fewer vaccinated people than unvaccinated people among the Covid-19 cases.

How does J&J's vaccine work?

The vaccine uses a harmless type of virus, called an adenovirus, which can cause cold symptoms. It is modified to contain the DNA of the so-called spike protein found on the surface of the new coronavirus. Once injected, the adenovirus carries the DNA payload into human cells. Once inside the cells, the DNA payload causes the production of the spike protein. This, in turn, triggers an immune response that can later defend against the real coronavirus if a vaccinated person is exposed to it.

What don't we know about J&J's vaccine?

We don't know its safety and effectiveness in children, or among pregnant women, their fetuses or nursing babies. We also don't know how long protection from the vaccine will last. The company is conducting a separate study testing whether adding a second dose improves its performance.

Does J&J's vaccine protect against new coronavirus strains?

The vaccine was less effective in South Africa and Latin America than in the U.S. That could be a sign the vaccine is less protective against strains circulating in those regions, though J&J is still conducting that analysis. Even at the lower level of effectiveness in South Africa and Latin America, health experts say, J&J's vaccine works well. The company said it is working on a version of the vaccine targeting the South Africa variant.

Write to Peter Loftus at peter.loftus@wsj.com

(END) Dow Jones Newswires

01-29-21 1228ET