What you need to know about the J&J vaccine

Overall, the vaccine prevented moderate to severe Covid-19 by 66.1 percent 28 days after the shot, but this rose to 85.4 percent when considering only severe disease

WASHINGTON (AFP) — European regulators are reviewing side effects of Johnson & Johnson's single-dose Covid-19 shot, after a handful of cases of rare blood clots were reported among its recipients.

Here's what you need to know about the vaccine:

How safe and effective is it?

Perhaps the most important figure to be aware of is "zero."

That's how many people have died from Covid-19 after receiving the shot in a massive global trial of 40,000 people aged 18 and over, compared to seven who got the placebo.

Overall, the vaccine prevented moderate to severe Covid-19 by 66.1 percent 28 days after the shot, but this rose to 85.4 percent when considering only severe disease.

What makes it most striking was that the good results against severe disease held up in both South Africa and Brazil, where concerning variants were dominant during the trial period.

In terms of safety, an analysis of 44,000 people showed it was generally well tolerated, with typical side-effects like injection-site pain, headache, fatigue and muscle pains.

In late February, the US pharma giant said that at least one case of anaphylaxis (severe allergic reaction) had been reported — and such reactions have also been recorded for other Covid-19 vaccines in rare instances.

The US Centers for Disease Control and Prevention (CDC) has advised people not to take the Johnson & Johnson shot if they have any history of severe allergic reactions.

The European Medicines Agency (EMA) said Friday it is reviewing side effects of the shot, after four cases, one fatal, of rare clots with low blood platelets were reported among its recipients.

The EMA said "it is not yet clear whether there is a causal association" between Johnson & Johnson's vaccine and the clots.

How does it compare to other authorized vaccines?

The Pfizer and Moderna RNA-based vaccines were shown to have efficacies of around 95 percent against all forms of Covid-19 — but it's still hard to make a direct comparison.

That's because their clinical trials took place before newer variants, some of which have been shown to be less susceptible to current vaccines, were well established.

The RNA vaccines probably still have an edge,

TIMOTHY D. EASLEY-POOL /AGENCE FRANCE-PRESSE FOLLOWING the approval of a third vaccine in the United States, an employee with the McKesson Corporation packs a box of the Johnson and Johnson Covid vaccine into a cooler for shipping from their facility in Shepherdsville, Kentucky.