By Peter Loftus 

Johnson & Johnson's Covid-19 vaccine was authorized for use in the U.S. by federal health regulators in late February. Authorities on April 13 recommended pausing its use to investigate rare but severe cases of blood clots.

It was the third shot to be cleared after vaccines from Pfizer Inc. and its partner BioNTech SE and from Moderna Inc. And it was the first shot requiring just one dose, rather than two. Here's what we know and don't know:

Why is the J&J vaccine being paused?

U.S. health authorities are investigating six reported cases of blood clots, including one death. Six women, ages 18 to 48 years, were reported to have experienced the clots as well as low levels of platelets, which help with clotting. The cases were extremely rare -- more than 6.8 million doses of the shot have been given in the U.S. so far -- but the U.S. Food and Drug Administration said it was making the recommendation out of an abundance of caution.

How do the three Covid-19 vaccines compare?

They all work well at preventing the Covid-19 disease, health experts say. The Pfizer-BioNTech and Moderna shots were more than 94% effective at preventing Covid-19 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. The J&J vaccine was 72% effective in the U.S. portion of its study.

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 hospitalization and death.

Which Covid-19 vaccine should I get?

Health authorities have generally said people should take the first vaccine they can get because early supplies have been constrained. But 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.

The U.S. Food and Drug Administration says people who are allergic to the ingredients of a vaccine shouldn't take it. The vaccines from Pfizer and Moderna contain, among other ingredients, the genetic material RNA and a substance called polyethylene glycol, which has been associated with rare allergic reactions. The J&J vaccine contains an adenovirus, a virus that can cause the common cold, which is rendered harmless; polysorbate and other ingredients.

A healthy, younger woman with a history of an allergic reaction known as anaphylaxis may opt for J&J's vaccine because there appears to be a lower rate of such side effects than seen for the vaccines from Pfizer and Moderna, said Dr. Gregory Poland, a Mayo Clinic vaccine researcher and fellow with the Infectious Diseases Society of America. Conversely, a 65-year-old man with a blood type associated with higher risk of severe Covid-19 may opt for a Pfizer or Moderna vaccine because of the strong evidence of efficacy in both clinical trials and real-world use, he said.

When can I get J&J's shot?

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

It appears to work well. The vaccine's 66% effectiveness rate shows it can protect a majority of 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. Now, health authorities are investigating the blood clots. A panel of outside experts will meet Wednesday to review the matter for the CDC, while the FDA will also conduct an investigation.

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 has started a study testing the vaccine in adolescents and 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. Health authorities are also probing the clotting issue.

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.

Have there been production problems with the J&J vaccine?

In late March, J&J said a batch of the main ingredient for its vaccine that was manufactured at a contractor's plant in Baltimore didn't meet quality standards. But no finished doses from the batch were distributed, and the contractor, Emergent BioSolutions Inc., plans to dispose of the batch. J&J says it still expects to meet its supply commitments for the vaccine.

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(END) Dow Jones Newswires

April 13, 2021 14:18 ET (18:18 GMT)

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