Elderly Britons jabbed with AstraZeneca's vaccine are less likely to have Covid antibodies than those who given Pfizer's, a study has suggested.
Imperial College London experts found that fewer than 85 per cent of over-80s had detectable levels of the virus-fighting proteins two weeks after their second AZ jab.
By contrast, the proportion of over-80s with antibodies after getting the second Pfizer vaccine was almost 98 per cent.
The findings came from Britain's largest surveillance study, known as REACT-2, which randomly tests blood samples from hundreds of thousands of Britons.
Although antibodies are just one part of the overall immune response to Covid, experts said they were not totally surprising.
Professor Ian Jones, a virologist at Reading University, told MailOnline the British jab was less likely to spark immunity because it relied on a weakened cold virus.
In some cases the body may attack this virus instead of the Covid proteins on its surface, which results in the jab failing to spark Covid immunity, he said. But Pfizer's jab does not have this problem because it uses a completely different technology.
In trials of the jabs, AstraZeneca's vaccine was also found to be slightly weaker at preventing symptomatic Covid infection.
But real-world analysis of Britain's vaccine rollout has shown that both vaccines are extremely effective at stopping severe illness and death.
Even against the Indian variant, they were both shown to reduce the risk of being hospitalised with the virus by more than 90 per cent.
Britain's largest study of Covid antibodies suggests Pfizer's jab is more likely to spark immunity to the virus than AstraZeneca's. Experts said this is because the Oxford-made jab relies on a weakened cold virus
REACT-2 looked at more than 207,000 participants from across the country for Covid-fighting antibodies.
The study was carried out over the two weeks to May 25 when the inoculation drive had only just been opened to people in their thirties.
Some individuals in younger age groups will have been vaccinated if they were in a top priority group — including healthcare workers and those who were clinically vulnerable to the virus because of an underlying condition.
