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What does COVID-19 vaccine effectiveness mean?

What does vaccine effectiveness mean? AP Illustration/Peter Hamlin

What does COVID-19 vaccine effectiveness mean?

It refers to the likelihood that a coronavirus shot will work in people.

Two vaccine makers have said that preliminary results from their late-stage studies suggest their experimental vaccines are strongly protective. Moderna this week said its vaccine appears nearly 95% effective. This comes on the heels of Pfizer’s announcement that its shot appeared similarly effective.

Those numbers raised hopes around the world that vaccines could help put an end to the pandemic sometime next year if they continue to show that they prevent disease and are safe.

Effectiveness numbers will change as the vaccine studies continue since the early calculations were based on fewer than 100 COVID-19 cases in each study. But early results provide strong signals that the vaccine could prevent a majority of disease when large groups of people are vaccinated.

U.S. health officials said a coronavirus vaccine would need to be at least 50% effective before they would consider approving it for use. There was concern that coronavirus vaccines might be only as effective as flu vaccines, which have ranged from 20% to 60% effective in recent years.

The broad, early effectiveness figures don’t tell the whole story. Scientists also need to understand how well the vaccine protects people in different age groups and demographic categories.

For both vaccines, the interim results were based on people who had COVID-19 symptoms that prompted a virus test. That means we don’t know yet whether someone who’s vaccinated might still get infected — even if they show no symptoms — and spread the virus.

Also unknown is whether the shots will give lasting protection, or whether boosters will be required.

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The AP is answering your questions about the coronavirus in this series. Submit them at: FactCheck@AP.org.

Read previous Viral Questions:

How does the coronavirus affect the heart?

Does the flu vaccine affect my chances of getting COVID-19?

Am I immune to the coronavirus if I’ve already had it?


How To Prevent the Spread of COVID-19 Coronavirus

COVID-19 coronaviruses transmitted from person to person. It is a virus that is similar to the common cold and the flu. So, to prevent it from spreading:

  • Wash hands frequently and thoroughly, with soap and water, for at least 20 seconds.
  • Wear a mask.
  • Don’t touch your face.
  • Keep children and those with compromised immune systems away from someone who is coughing or sneezing (in this instance, at least six feet)
  • If there is an outbreak near you, practice social distancing (stay at home, instead of going to the movies, sports events, or other activities.)
  • Get a flu shot.

Local resources

KSL Coronavirus Q&A

Utah’s Coronavirus Information

Utah State Board of Education

Utah Hospital Association

The Church of Jesus Christ of Latter-day Saints

Utah Coronavirus Information Line – 1-800-456-7707

National Resources

Centers for Disease Control and Prevention

Commonly asked questions, World Health Organization

Cases in the United States