A single dose of the Pfizer-BioNTech or University of Oxford-AstraZeneca coronavirus vaccine produces a "similar" antibody response in people aged 80 or over, research suggests.
The UK has three jabs in its immunisation arsenal – Pfizer-BioNTech, Oxford-AstraZeneca and Moderna, with the latter only recently being rolled out.
The two-dose Pfizer-BioNTech and Oxford-AstraZeneca vaccines were approved after being found to be 95% and 70% effective at warding off severe disease in clinical trials, respectively.
This was muddled, however, with the Oxford-AstraZeneca jab found to be up to 90% effective when administered as a half dose followed by a full dose. The team behind the vaccine later announced no one who received any dose of the jab had been hospitalised with the coronavirus in a late-stage trial.
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Scientists from the University of Birmingham have now revealed the vaccines initiate a similar antibody response around five weeks after the first jab in people aged 80 to 99.
The Oxford-AstraZeneca vaccine may have the edge when it comes to other aspects of the immune response, however, with the jab found to produce stronger T-cell levels.
This comes after scientists from the University of Bristol reported a single dose of the Pfizer-BioNTech or Oxford-AstraZeneca jab is 79% or 80% effective, respectively, at warding off severe disease in the elderly after 14 days.
The preliminary results have been published via Preprints with The Lancet and are yet to appear in a peer-reviewed journal.
"It is important to understand how the immune response generated by COVID-19 [the disease caused by the coronavirus] vaccines varies with age, the delay between doses and the type of vaccine administered," said study author Professor Paul Moss.
"As far as we know, this study is the first of its kind to compare both antibody and T-cell responses following a single dose of either the Pfizer or AstraZeneca vaccine in any age group.
"The findings are reassuring because many countries, including the UK, have chosen to delay administering second doses."
Both the Pfizer-BioNTech and Oxford-AstraZeneca vaccines were approved as two-dose regimens, with an interval of three to four weeks between the jabs being administered.
With research strongly suggesting a robust immune response after the first doses, the UK's vaccination strategy shifted towards giving more people an initial jab, followed by a second vaccine up to 12 weeks later.
Many experts are confident this approach has saved lives. It may even "prime" the immune system between doses, leading to greater protection after the second jab is administered.
To better understand the immune effects of giving just one dose, the Birmingham scientists analysed 165 people aged 80 to 99 who lived independently, in a study partially supported by the UK Coronavirus Immunology Consortium.
Elderly people were recruited due to them being more at risk of coronavirus complications and the immune response, both natural and post-vaccine, declining with age.
The two vaccines were also approved based on trials with relatively few elderly participants. This prompted German officials to initially recommend the Oxford-AstraZeneca jab only be given to people under 65.
Of the 165 participants, 76 had received one dose of the Pfizer-BioNTech vaccine, while the remaining 89 had the first Oxford-AstraZeneca jab.
Blood samples were collected five to six weeks after the doses were administered.
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Results reveal 93% of those who had the Pfizer-BioNTech vaccine and 87% who received the Oxford-AstraZeneca jab had "spike-specific" antibodies.
The coronavirus uses its so-called spike protein to bind to a receptor present on many cells, allowing it to invade the body.
"Similar levels of antibody response were found after both vaccines," wrote the scientists.
All the participants had a detectable antibody response after their second dose, highlighting the importance of taking up both vaccines when called, added lead author Dr Helen Parry.
Fewer of the participants had T-cells after the first dose. These were detectable in just 12% of the Pfizer-BioNTech and 31% of the Oxford-AstraZeneca participants, respectively.
"The strength of this cellular response was also three times higher following the AstraZeneca vaccine when compared with the Pfizer vaccine," wrote the scientists.
Antibodies are proteins that are specific to a pathogen, like the coronavirus. They lock onto the virus' surface, neutralising or "marking" it for destruction by other immune cells.
When it comes to T-cells, there are two types: helper and killer.
Helper T-cells stimulate antibody production and assist in the development of killer cells. Killer T-cells directly destroy body cells that have already been infected.
T-cells also send out messages instructing the rest of the immune system to ramp up its response.
The antibody response is said to be more affected by the emergence of new coronavirus variants.
"Antibodies are like a lock and a key," said Professor Moss. "A single change to [the spike protein's] amino acids [the building blocks of a protein] means a lot of antibodies can't bind."
T-cells, however, are arranged more like a "chain", making them "less susceptible to [the] loss of immune recognition to variants".
Professor Moss has stressed the higher T-cell response observed in the Oxford-AstraZeneca participants does not necessarily mean that vaccine performs better against new coronavirus variants.
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The Birmingham scientists have also reported eight of the participants showed signs of having overcome the coronavirus naturally pre-vaccination.
These individuals' antibody levels were 691 times higher than those who only had a first jab, with their T-cells being elevated four-fold, the results show.
"It's a remarkable priming effect," said Professor Moss. This may be particularly potent among elderly patients due to their vaccine response being smaller, he added.
The scientists concluded: "These results indicate antibody responses develop in most people over 80 years of age at five weeks after a single dose of either the Pfizer or AstraZeneca COVID vaccine.
"This reassuring level of antibody immunity from a single dose of either vaccine is likely to underpin the encouraging clinical protection seen with these vaccines.
"The relatively lower rates of T-cell response observed may reflect the reduced immune function that is generally observed in older people.
"It remains to be seen if the different levels of T-cell response recorded after each vaccine will have any impact on clinical effectiveness."