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

"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.