Study confirms AstraZeneca's vaccine is moderately effective

Peer-reviewed research published Tuesday confirms that the coronavirus vaccine developed by AstraZeneca and Oxford University is moderately effective at preventing infection, and it appears to keep people out of the hospital.

Why it matters: Some questions remain about AstraZeneca's vaccine, but the findings released in The Lancet medical journal help validate that the product works and is safe.

The big picture: When AstraZeneca's vaccine was administered in two standard doses, it was about 62% effective in preventing symptomatic infections.

• That's not as high as the roughly 95% efficacy reported by the Pfizer and Moderna vaccines, but AstraZeneca's has some other advantages over those two, including easier distribution.
• No one who had received both doses of the vaccine in AstraZeneca's trials developed a severe infection or had to be hospitalized.

The big question: The company accidentally gave some patients a modified dosing regimen — a half dose, followed by a full standard dose. In those patients, the vaccine registered 90% efficacy.

• Experts have puzzled over those results since AstraZeneca's initial release, and the research published Tuesday doesn't have many answers.
• The group that got the half-dose didn't include anyone older than 55 — the people most at risk for severe infection. We still don't know whether that skewed the results or AstraZeneca accidentally hit on a better dosage.

The lower initial dose also appeared to be much more effective in preventing asymptomatic spread — a critically important part of stopping the virus from spreading.