The booster dose elicited a significantly higher antibody response against the initial strain of coronavirus and the Delta and Beta variants, compared to what was seen among people who got two doses.
The booster dose seemed to be equally protective against the Delta and Beta variants as against the original coronavirus.
"Given the high levels of immune responses observed, a booster dose given within six to 12 months after the primary vaccination schedule may help maintain a high level of protection against COVID-19," the company said in a statement.
"This initial data indicate that we may preserve and even exceed the high levels of protection against the wild-type virus and relevant variants using a third dose of our vaccine," added Dr Ugur Sahin, CEO and co-founder of BioNTech.
"A booster vaccine could help reduce infection and disease rates in people who have previously been vaccinated and better control the spread of virus variants during the coming season."
The data on booster shots comes as COVID-19 cases and hospitalisations have surged in recent weeks, particularly among unvaccinated people in the South.
Pfizer executives say they believe a booster dose will be needed soon, but US health officials say they have seen no indication one is needed yet.
Last week, the FDA and Centres for Disease Control and Prevention authorised and recommended a third dose of vaccine for some immunocompromised people who likely had little or no response to the first two shots.
Still, health officials have focused on persuading unvaccinated people to get protected.
As of Monday (local time), 50.7 per cent of the US population was fully vaccinated, including 59.3 per cent of those eligible, according to data from the CDC.
Cases are surging in 40 states, according to data from Johns Hopkins University.
Soon, the US could be reporting more than 200,000 new cases a day, the director of the National Institutes of Health predicted on Sunday (local time).
That's a rate not seen since before vaccines became widely available.
"That's heartbreaking considering we never thought we would be back in that space again," Dr Francis Collins said on Fox News.
Pfizer and BioNTech said they expect results from the trial to evaluate a booster dose "shortly" and the companies plan to submit the data to the FDA and other regulatory authorities.
The companies plan to submit this early data to the European Medicines Agency and other regulatory authorities in the coming weeks.
Pfizer and BioNTech said after FDA grants approval for their vaccine in the US, they would then seek approval of a booster dose through a supplement to their license application.
The companies said they would seek approval for this booster dose for people 16 or older.
Currently, the Pfizer COVID-19 vaccine is authorised for people 12 and older.
Texas tops US in number of COVID-19 pediatric hospitalisations
Texas leads the nation in the number of current confirmed COVID-19 pediatric hospitalisations, the latest data from US Health and Human Services shows.
Texas, with 239 children hospitalised with COVID-19, has surpassed Florida, which has 170 children with coronavirus in hospital care.
Florida, which is third, has the country's highest total number of adults and children newly admitted into the hospital with cases of COVID-19.
The latest HHS data shows 2061 adults and 59 children were admitted with COVID-19 in Florida hospitals since the previous day.
As for total adult COVID-19 hospitalisations, Florida has the most with 15,486 patients, followed by Texas with 11,083, according to the latest HHS data.
Surgeon General wary of fading precautions
On Sunday (local time), the surgeon general said many of those who have followed precautions may soon grow complacent due to fatigue.
"I do think that many of those who did make that right decision to get vaccinated, are thinking, 'Gosh, I did the right thing but here we are, still in the middle of this pandemic 18 months later,'" Surgeon General Dr Vivek Murthy told CNN's Brian Stelter.
"We get through this pandemic when a critical majority of us get vaccinated, and we haven't hit that critical number yet," Dr Murthy said.
Dr Murthy said he is especially worried about parents who have been working hard to protect their children as the nation grapples with the pandemic.
That fear is especially pressing as more children are being hospitalised with the virus, such as Kyle Butrum's one-year-old son, Carter, in Arkansas.
Mr Butrum urged people to get vaccinated so other families don't have to watch their child struggle in the hospital.
"The only thing you can do to prevent someone else from doing this is to get your vaccine, so that another child doesn't have to do this, and another family doesn't have to send their kid away. So another father doesn't have to stand at the back of the ambulance and wonder if that's the last time you're going to see your son," Mr Butrum told CNN.
With the rise of Delta variant and the start of the new school year, CNN medical analyst Dr Leana Wen said this is the most dangerous time in the pandemic for children.
Experts have shared the best practices to keep families safe, but "we as a society have failed our children," Dr Wen said, citing the swathes of people navigating communities unmasked and unvaccinated and politicians disregarding the guidance.
"As a result, we have made it harder for schools to stay open, we have made it more likely for children to be infected," Dr Wen said.
"This is a really sad reflection that children are having to pay the price for irresponsible adults and reckless politicians."
Officials at odds over masking
Many health experts have pointed to mask mandates as a necessary step to getting the pandemic under control as officials work to boost vaccination rates, but some local leaders are increasingly at odds over such mandates.
Texas Attorney General Ken Paxton took the fight against mask mandates in his state to the Texas Supreme Court after various defeats in lower courts.
The state Supreme Court ruled Sunday in favor of Paxton and Texas Governor Greg Abbott, temporarily blocking local mask mandates, but officials in Dallas and San Antonio said they would continue to enforce school mask mandates, despite the court ruling.
Mr Abbott and Florida Governor Ron DeSantis were the recipients of letters from the Department of Education in which Education Secretary Miguel Cardona said he is "deeply concerned" over mask mandate restrictions.
"The Department recognises that several school districts in your state have already moved to adopt such policies in line with guidance from the CDC for the reopening and operation of school facilities despite the state-level prohibitions."
The Department stands with these dedicated educators who are working to safely reopen schools and maintain safe in-person instruction," Mr Cardona wrote in the letter to DeSantis, which was also addressed to Florida Education Commissioner Richard Corcoran.
In May, Mr DeSantis issued two executive orders limiting the COVID-19 restrictions counties and cities could impose, including mask mandates, and has since threatened punitive measures should a county or city defy them.
In an email to CNN on Sunday, a spokesperson for Mr DeSantis reiterated the governor's position that his ban on school mask mandates is about empowering parents to make the best choice for their child and family.
The Broward County School Board chair criticised Mr DeSantis on Sunday over his position on school mask mandates.
"We believe we have a constitutional obligation to protect the lives of our students and staff. And we've received threats from our governor, and it's been really, really dramatic and horrible to be put in this position," Dr Rosalind Osgood said on CBS' "Face the Nation" on Sunday.