First dose of AstraZeneca, Pfizer COVID-19 shots get equally strong antibody response from elderly

The UK study also found that a critical component of the immune system known as T cells showed a more enhanced response in those who got the AstraZeneca/Oxford University vaccine than in those who got the Pfizer/BioNTech one.

The scientists behind the study, which analysed blood samples from 165 people in Britain aged between 80 and 99 who had been given a first COVID-19 vaccine dose, said that finding merited further investigation.

While antibodies can block the coronavirus' ability to enter human cells, T cells can act as broader protection by attacking and killing any cells that have been infected with the virus.

"These vaccines are both equivalent and effective at inducing antibody responses in the great majority of people (in this study), even after one dose," said Paul Moss, a professor of haematology at Britain's Birmingham University who co-led the research.

"Both vaccines are good," he told a briefing on the findings.

The researchers also found that immune responses were "markedly enhanced" by prior infection with SARS-CoV-2. Across the whole cohort of 165, evidence of previous natural infection was seen in 8 donors and associated with 691-fold and 4-fold increase in humoral and cellular immune responses, according to the report.

The study was published online as a Lancet preprint before being peer reviewed. It found that key antibodies were present and at similar levels in 93% of the 76 study participants who got the Pfizer single COVID-19 vaccine dose, and 87% of the 89 participants who got the AstraZeneca shot.

Helen Parry, clinical lecturer at Birmingham University who co-led the study, said the tests also showed that a larger proportion of those who got the AstraZeneca vaccine had a detectable T cell response.

That finding adds to other emerging evidence from COVID-19 studies suggesting that antibodies - which stop the coronavirus from being able to attach to human cells - may not be the only important element of immunity, and that T-cells may also have an important protective role.

"We now need to carry out further research to understand what this difference in T cell responses means," said Parry.

Around half of the UK population have had at least one dose of a COVID-19 vaccine, one of the highest rates of inoculation in the world. The vast majority have received either the Pfizer or the AstraZeneca shot.