Study confirms AstraZeneca's vaccine is moderately effective

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.