A booster shot of the Pfizer-BioNTech vaccine is 95.6% effective against the coronavirus, compared with those who only had the first two initial doses, according to a large study released Thursday.

Why it matters: The study, which had more than 10,000 participants aged 16 and older, is the first controlled, randomized trial looking at boosters and their effectiveness, the companies said.

The companies said the study was conducted during a time when the Delta variant was “the prevalent strain.”

Details: Trial participants received either a booster shot or a placebo. “The median time between second dose and administration of the booster dose or placebo was approximately 11 months,” the companies said.