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The biotech giant Pfizer released promising early data from its study of an experimental COVID-19 vaccine. Here are some answers to key questions about the development.

Why is the Pfizer announcement stirring so much excitement?

This is the first report from a late-stage trial of a potential coronavirus vaccine, among the four being tested in large trials in the United States.

Preliminary data show that the vaccine made by Pfizer and the German company BioNTech appears to be 90 percent effective in protecting people against COVID-19. That’s an extremely high rate, much better than the flu vaccine. The Food and Drug Administration is requiring only that coronavirus vaccines be 50 percent effective.

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What does it mean to be 90 percent effective?

In a study that enrolled 44,000 people, half received two doses of the vaccine and half two doses of a placebo. Then they went about their daily lives.

So far, 94 study participants have contracted COVID-19. Ninety percent of those who got sick were people who had taken the placebo. That suggests that most of those who got the vaccine were protected by it.

Why is this finding considered preliminary?

Pfizer released few details about its research, and only in the form of a press release, rather than a peer-reviewed article. Its data have not yet been scrutinized by the FDA or its expert panel.

Among the uncertainties:

  • The study isn’t over. Pfizer plans to continue until 164 COVID-19 cases emerge.
  • The 90 percent figure could change as the trial continues.
  • No one knows how long the vaccine’s protection will last.
  • It’s not clear whether the vaccine prevents severe cases of COVID-19.
  • There is no information on whether it prevents people from carrying the virus without symptoms.

How does the vaccine work?

The technology powering the vaccine — known as messenger RNA or mRNA — has never been used in an approved drug.

Traditional vaccines employ a weakened or killed virus to stimulate an immune response.

Pfizer’s vaccine inserts portions of the coronavirus’s genetic material (RNA) into cells, prompting the cells to manufacture a piece of the virus. The viral particle then triggers the production of antibodies against the virus, potentially protecting people from infection.

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Cambridge-based Moderna Inc. is using the same approach with its experimental COVID-19 vaccine, which is also in a late-stage clinical trial.

What’s next?

Pfizer still needs to finish collecting the required two months of safety data, which it expects to complete later this month. Then it will ask the Food and Drug Administration for emergency authorization to distribute the vaccine. The company intends to have manufactured enough doses to immunize 15 to 20 million people by the end of the year.

If the FDA allows the vaccine to be distributed, there remain many challenges. It requires two doses three weeks apart to be effective. In addition, it has to be kept extremely cold and then defrosted before being administered.

Jonathan Saltzman of the Globe staff contributed to this report. Material from The New York Times and STAT was included.


Felice J. Freyer can be reached at felice.freyer@globe.com. Follow her on Twitter @felicejfreyer.