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What does the effectiveness of a COVID-19 vaccine mean?

What does the effectiveness of a COVID-19 vaccine mean?

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

Two manufacturers have said that preliminary results from their late-stage studies suggest that their experimental vaccines offer strong protection. Moderna indicated this week that hers has an apparent effectiveness of almost 95%, after a few days ago Pfizer announced that its vaccine appeared to be similarly effective.

Those figures raised hopes around the world that vaccines could end the pandemic sometime next year if they continue to show that they prevent people from contracting the disease and are safe.

The effectiveness figures will be modified as the vaccine studies continue, since the first estimates were based on fewer than 100 cases of COVID-19 in each study. But early results provide strong indications that the vaccine could prevent the vast majority of infections when large groups of people are vaccinated.

US health authorities noted that a coronavirus vaccine would have to be at least 50% effective before they would consider approving it for use. There was concern that vaccines for COVID-19 would have similar efficacy to influenza vaccines, whose effectiveness has ranged between 20% and 60% in recent years.

Preliminary figures do not provide the complete picture. Scientists also need to understand how well the vaccine protects people in different age groups and demographic categories.

For the two vaccines, the interim results were obtained in people who had symptoms of COVID-19 and for whom they were given a diagnostic test. That means we don’t yet know if someone who is vaccinated could still become infected — even if they show no symptoms — and spread it to others.

It is also not known whether the vaccines will provide lasting protection, or whether boosters will be necessary.