How effective is the Johnson & Johnson vaccine? What to know

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The first 4 million doses of Johnson & Johnson's Covid-19 vaccine are rolling out this week, joining vaccines from the drugmakers Moderna and Pfizer-BioNTech, which have been in use since December.

Johnson & Johnson's single-dose shot, made in partnership with Janssen Pharmaceuticals, differs from the other two in several ways. It's made differently and, at first glance, might appear to be less effective.

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But that does not necessarily mean the Johnson & Johnson vaccine is inferior. Here's why:

How effective is the Johnson & Johnson vaccine?

When companies report results from clinical trials, they report the "percent effectiveness" of a vaccine. A vaccine that's 90 percent effective at preventing illness does not mean that 10 percent of people who received it will get sick; rather, it means that people who received the vaccine were 90 percent less likely to get sick, compared with people who received the placebo.

In Johnson & Johnson's trial, researchers looked at different outcomes in different parts of the world. In the United States, for example, the vaccine was found to be 72 percent effective at preventing what the company defined as moderate to severe Covid-19. That was a broad definition, which ranged from a combination of milder symptoms, such as fever and headache, to more severe symptoms, such as shortness of breath and low oxygen levels.

That effectiveness varied when researchers tested the vaccine in other countries, where variants of the virus are circulating. In Latin America, where the variant P.1 has cropped up, the vaccine was found to be 66 percent effective. Studies conducted in South Africa, where a variant called B.1.351 is circulating, effectiveness was lower: 64 percent.

But these numbers don't tell the whole story.

When researchers looked specifically at the vaccine's protection against the most severe forms of illness, the effectiveness shot up to 86 percent.

And it prevented 100 percent…
Erika Edwards, Denise Chow
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