Pfizer Inc and German partner BioNTech SE on Thursday said data from a Phase III trial demonstrated high efficacy of a booster dose of their COVID-19 vaccine against the virus, including the Delta variant.

They said a trial of 10,000 participants aged 16 or older showed 95.6% effectiveness against the diseases, during a period when the Delta strain was prevalent.

The study also found that the booster shot had a favorable safety profile.

Pfizer had said its two-shot vaccine's efficacy drops over time, citing a study that showed 84% effectiveness from a peak of 96% four months after a second dose. Some countries had already gone ahead with plans to give booster doses.

The drugmakers said the median time between the second dose and the booster shot or the placebo in the study was around 11 months, adding that there were only five cases of COVID-19 in the booster group, compared with 109 cases in the group which received the placebo shot.

"These results provide further evidence of the benefits of boosters as we aim to keep people well-protected against this disease," Pfizer CEO Albert Bourla said in a statement.

The median age of the participants was 53 years, with 55.5% of participants between 16 and 55 years, and 23.3% at 65 years or older.

The companies said they would submit detailed results of the trial for peer-reviewed publication, to the U.S. Food and Drug Administration (FDA), the European Medicines Agency, and other regulatory agencies as soon as possible.

U.S. and EU regulators have already authorized a third dose of COVID-19 vaccines by Pfizer-BioNTech and Moderna Inc for patients with compromised immune systems who are likely to have weaker protection from the two-dose regimens.

(Reporting by Riham Alkousaa; editing by Jason Neely)