Pfizer Inc. and BioNTech SE said a booster shot of their COVID-19 vaccine restored full protection in a large study, results that are likely to bolster the argument for giving a third dose more widely. 

A booster was 95.6% effective against symptomatic COVID in the study, which followed 10,000 people aged 16 and older, the companies said in a statement Thursday. The fast-spreading delta variant was the predominate strain during the trial.

"We believe boosters have a critical role to play in addressing the ongoing public health threat of this pandemic," Pfizer Chief Executive Officer Albert Bourla said in a statement. The companies said they'll share the data with health authorities in the U.S., Europe and elsewhere.

Regulators have wrestled with how widely to use boosters as the delta variant drives infection rates up. Some countries, such as Israel, are using boosters extensively. Many others -- including the U.S. and much of Europe -- have thus far come down on the side of a third dose for the elderly and other high-risk individuals. Exactly where to draw the line on who is high-risk has also been a matter of debate.

Half the trial participants got a booster, with five cases in that group over an average follow-up period of two and a half months. There were 109 cases in the group randomly assigned a placebo shot. The booster was just as safe as the original two-dose vaccine.

The trial results show that "booster vaccinations could play an important role in sustaining pandemic containment and a return to normalcy," BioNTech CEO Ugur Sahin said.

Trial participants received a booster an average of 11 months after their second dose of the vaccine. They were an average of 53 years old, with slightly under one-quarter of the group older than age 65. Efficacy of the third shot was consistent across age groups and among people with pre-existing conditions, the companies said.

The U.S. Food and Drug Administration said this week that older or high-risk people who got Moderna Inc.'s treatment can also get an extra shot, as can all adults who took the one-dose Johnson & Johnson vaccine. The agency backed mix-and-match boosters, saying that each of the available COVID shots can be used, no matter which vaccine the recipient got the first time.