By Peter Loftus 

Johnson & Johnson's Covid-19 vaccine was authorized for use in the U.S. by federal health regulators. It is the third shot to be cleared after shots from Pfizer Inc. and its partner BioNTech SE and from Moderna Inc. And it is the first shot requiring just one dose, rather than two. Here's what we know and don't know:

How do the three Covid-19 vaccines compare?

They all work well, health experts say. The Pfizer-BioNTech and Moderna shots were more than 94% effective in their late-stage trials, while J&J's effectiveness was 66%. But the difference may be less than it seems. The Pfizer-BioNTech and Moderna shots were tested before the emergence of strains like one first identified in South Africa, which various studies have indicated eludes vaccines somewhat, though they still work against it. J&J's vaccine was tested in South Africa, and its effectiveness wasn't as high there as in other countries, hurting the vaccine's overall effectiveness. Yet health experts say J&J's vaccine is plenty effective. Importantly, it was 85% effective against severe Covid-19 at least 28 days after vaccination. That suggests the vaccine should help many people avoid the kinds of serious cases that result in hospitalizations.

Which Covid-19 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 is given as a single dose, which may be a more convenient option than the two doses required for both of the Pfizer-BioNTech and Moderna vaccines, given three or four weeks apart. Only the Pfizer-BioNTech vaccine is authorized for adolescents 16 and 17 years old, while J&J and Moderna's shots are cleared for those 18 and older.

When can I get J&J's shot?

J&J has been making doses while testing its shot, so it will be able to ship nearly four million doses within a few days. Vaccinations will be limited at first, however. The company plans to deliver 20 million doses for U.S. use by the end of March. Then production will really pick up, as J&J projects shipping 100 million doses by the end of June.

How effective is J&J's Covid-19 vaccine?

It appears to work well. The vaccine's 66% effectiveness rate shows it can protect most vaccinated adults from moderate to severe Covid-19, the disease caused by the coronavirus. And the vaccine was even more effective preventing severe disease specifically, posting the 85% rate. By comparison, an annual flu shot is considered to work well if it is 60% effective.

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

The most common side effects among people who received the vaccine were injection-site pain, headache and fatigue. Most were mild to moderate. J&J has received preliminary reports of two cases of severe allergic reactions in vaccine recipients, one of which was anaphylaxis.

How was J&J's Covid-19 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. The 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. Researchers also monitored for side effects among the study subjects.

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. After injection, the cold-virus vector 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 Covid-19 vaccine?

We don't know its safety and effectiveness in children, or among pregnant women, their fetuses or women nursing babies. We also don't know how long vaccine-induced protection will last. The company is conducting a separate study testing whether adding a second dose improves its performance. Also uncertain is whether the vaccine can help curb the spread of the virus by infected people who don't have symptoms, a group that turns out to be responsible for a lot of transmission. A preliminary analysis by J&J gave tantalizing signs the company's vaccine could limit asymptomatic spread, by reducing infections that didn't cause symptoms in study subjects.

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. J&J's vaccine was 57% effective in South Africa and 66% in Latin America, compared with 72% in the U.S. during the late-stage trial, according to J&J. Even at the lower levels 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 variant first identified in South Africa. Even in study subjects there, the shot still 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.

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

 

(END) Dow Jones Newswires

February 27, 2021 19:32 ET (00:32 GMT)

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