Extending the interval between doses of the BioNTech/Pfizer vaccine from three to 12 weeks strengthens the antibody response in elderly people, according to a UK study.

The researchers at Birmingham university, working in collaboration with Public Health England, said their findings vindicated the UK government’s controversial decision in December to wait 12 weeks between first and second doses rather than the 21 days recommended by Pfizer’s clinical trials.

“The advice to prioritise first doses, so as to provide a greater public health impact and save more lives, was considered quite bold at the time but the data shows that it has paid off,” said Gayatri Amirthalingam, a PHE epidemiologist and co-author of the study, which has not yet been peer-reviewed.

The Birmingham study involved 172 people aged 80-99, who each received two shots of the Pfizer vaccine. Of the group, 73 had a 12-week interval between doses and 99 a three-week interval. Peak levels of neutralising antibodies, reached 2-3 weeks after the second jab, were 3.5 times higher on average in the group with the extended gap.

Although the researchers do not know yet how long this increased antibody response will last or whether it will translate into enhanced protection against infection and disease, Paul Moss, professor of haematology at Birmingham, was optimistic.

“Our research findings may be important in the development of global vaccination strategy, as an extension of the interval of the second vaccine dose in older people may potentially reduce the need for subsequent booster vaccines,” he said.

The study also looked at the response of the cellular immune system to the different dosing regimen but the results were less clear.

The 3-week interval generated T-cells targeting the Sars-Cov-2 virus more quickly than the 12-week interval and reached a higher peak, but levels then declined. T-cell levels were similar 15 weeks after the first dose for both interval gaps.

“The significance of these cellular responses is not yet clear,” said Helen Parry of Birmingham, the first author of the paper. “We will follow up the study of these patients and see how the antibody and cellular data change over the next six months.”

Eleanor Riley, professor of immunology and infectious disease at Edinburgh university, who was not involved in the study, commented: “Overall, these data add considerable support to the policy of delaying the second dose of Covid-19 vaccine when vaccine availability is limited and the at-risk population is large.”

She added: “Longer-term follow-up of this cohort will help us to understand which vaccine interval will be optimal in the future, once the immediate crisis is over.”

In separate statement, PHE said on Friday that Covid-19 vaccinations in the UK had directly prevented an estimated 11,700 deaths of people aged 60 and older by the end of April, and kept 33,000 people aged 65 and older out of hospital.