Research by the University of Birmingham and Public Health England’s Porton Down laboratory tracked the immune response in people aged over 80.
Scientists said the vaccine delivered an “off the scale” immune response among recipients which meant it was likely to work against multiple variants of the virus.
The research, which is the biggest Pfizer study on antibody and cellular immune factors to date, found 98 per cent of people aged 80 or over who had two doses of the Pfizer jab had a strong antibody immune response.
A total of 100 people aged 80 to 96 were studied, having received their vaccine doses three weeks apart, before the UK adopted a policy of stretching the time between jabs to 12 weeks.
Published in The Lancet, the research found people who had previously had natural Covid-19 infection had a peak antibody response after just one Pfizer vaccination.
Blood samples from all participants showed the original Wuhan strain was strongly neutralised after two doses of the jab.
While neutralisation reduced 14-fold when tested against the Brazilian variant, experts believe the very high immune response generated by the vaccine is enough to provide protection against that strain also.
The study found cellular T cell responses developed in 63 per cent of the older people given two doses of the Pfizer jab.
Like the cells which produce antibodies, T cells are crucial to the immune system’s response to viral infection.
Professor Paul Moss, from the University of Birmingham and leader of the UK Coronavirus Immunology Consortium, told a briefing: “In terms of the variants and how we control them, people seem to be taking two broad approaches.
“We can develop another vaccine, a booster against these specific variants, so we perhaps have a South African booster vaccine.
“The other potential is that you boost your antibody levels so high from whatever vaccine you have that there’s enough to go around and you cope with the variant.
“We’ve certainly seen in this paper that the antibody levels are so good, really after the first two weeks, that we are pretty confident that this should be very helpful against the Brazilian variant.”
Asked if he is surprised how well the vaccines have worked in older people, he said: “We were. When we sent these samples to Porton Down they said, ‘We can’t give you results right now because we’ve got to dilute them because they’re so high, they’re off the scale’.
“The antibody levels were so high that they’d gone above the thresholds.”
But he added it will be crucial to see how long antibody levels are maintained.
He said : “It will be important to assess that and whether they wane at different rates in people of different ages.”
It comes as other new data from Pfizer suggests the vaccine is 100 per cent effective in preventing Covid-19 in South Africa.
Out of 800 people in the South African part of a Phase Three clinical trial, nine cases of Covid-19 were observed – all of whom were in the unvaccinated group.
Six of the nine were the South African strain. Pfizer says this demonstrates the jab can induce “a robust neutralising antibody response to the (South African) variant”.