Why are there varying reports of how well vaccines work?

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By , KCBS Radio

The FDA granted an emergency use authorization to the Johnson & Johnson vaccine over the weekend after experts concluded that the vaccine is highly effective.

But reports of just how well the vaccine works have varied from 66 percent to 72 percent to 85 percent, causing some confusion.

"They had several different endpoints and they had studies going on in different countries. So think of it as a kind of table," explained Dr. George Rutherford, Director of the Prevention and Public Health Group at UC San Francisco.

Johnson & Johnson tracked how well its vaccine worked at preventing asymptomatic infection, mild to moderate disease, critical and severe disease and death, as well as the results in the U.S. versus worldwide.

"So that’s why you’re seeing so many different percentages," he said.

The vaccine was found to be 66 percent effective at preventing moderate to severe cases of COVID-19 across the world, and 72 percent effective in the U.S. When looking at severe cases only, it was 85 percent effective.

Importantly, no one in the clinical trial who received the Johnson & Johnson vaccine has died from the virus.

"I tend to think of this as being between 80 percent to 85 percent effective, and that kind of gives us the benefit of the doubt for looking just at the U.S. data," said Dr. Rutherford. "Because they recruited a lot of patients in South Africa where these other variants are."

Dr. Rutherford added that people should not worry too much about the varying levels of efficacy reported between the Johnson & Johnson, Pfizer and Moderna vaccines.

"You don’t know it until you put them out there together and really race head to head, and so these vaccines were NOT compared head to head," he explained.

For example, the Johnson & Johnson vaccine trial was somewhat later in the pandemic after variants started to become more prominent.

He said as the vaccine is distributed widely, more data will be compiled to look at how the vaccines perform under the same conditions.

Many public health experts have said that all three vaccines available in the U.S. will significantly reduce your risk of becoming hospitalized or dying because of COVID-19, and people should take whichever vaccine becomes available to them.

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