A study conducted by two labs found a booster dose of Pfizer’s COVID-19 vaccine was 95.6% effective against symptoms of the disease.
The trial, conducted on 10,000 people over the age of 16, shows “a relative efficacy of 95.6%” and a “favourable safety profile”, according to a statement
The vaccine, developed by Pfizer and BioNTech, was administered as a third dose to people who had previously had two doses.
According to the companies, the booster dose “restored vaccine protection against COVID-19 to the high levels achieved after the second dose, showing a relative vaccine efficacy of 95.6% when compared to those who did not receive a booster”.
It was the first efficacy results from a randomised, controlled COVID vaccine booster trial.
The companies said they plan to submit the information to the US’s FDA, the European Medicines Agency (EMA) and other agencies.
"These results demonstrate once again the usefulness of boosters in our efforts to protect the population against this disease," said Albert Bourla, Pfizer's chief executive, quoted in the statement.
The median age of the participants was around 53 years.
Several countries have already allowed a booster dose to boost immunity in vaccinated people, which appears to decline after several months, according to some studies.
In the United States, experts from the FDA have been recommending a third Pfizer/BioNTechdose for certain at-risk populations, such as the over-65s, since late September.
"The available data suggest that immunity is declining in some fully vaccinated populations," the FDA's acting head, Janet Woodcock, recently explained.
In Europe, the EMA approved in early October the principle of a third dose of Pfizer/BioNTech for people over 18, leaving it up to states to decide which populations are eligible.
France, for example, has started to administer this booster dose to certain categories of the population: the elderly (six months after their vaccination) and people with weakened immune systems.
Other governments have gone further: in Israel, the third dose is available from the age of 12, five months after vaccination.
Meanwhile, the EMA said on Tuesday it was expecting to be ready to decide on recommending a booster jab of the Moderna vaccine by 25 October.
Marco Cavaleri, the EMA’s Head of Biological Health Threats and Vaccines Strategy, also gave an update on the agency’s view on so-called mix and match vaccine schedules.
Using a different vaccine for a booster to that used for the first two doses is already endorsed by the FDA.
Cavaleri said: “We are seeing some promising results from studies that confirm that this approach will trigger with some vaccine combinations a stronger immune response than when the same vaccine is used for an additional shot.
“As more evidence is being consolidated we will consider if such clinical data could be sufficient to expand the recommendation on the use of each vaccine.”
He also urged countries and individuals to continue getting vaccinated against the disease.
With close to five million people dead worldwide as a result of COVID-19, he said the cost of natural immunity - “in other words letting the virus run wild” - is not tolerable.