The UK’s decision to delay second doses of coronavirus vaccines has received fresh support from research on the over-80s which found that giving the Pfizer/BioNTech booster after 12 weeks rather than three produced a much stronger antibody response.
A study led by the University of Birmingham in collaboration with Public Health England found that antibodies against the virus were three-and-a-half times higher in those who had the second shot after 12 weeks compared with those who had it after a three-week interval.
Most people who have both shots of the vaccine will be well protected regardless of the timing, but the stronger response from the extra delay might prolong protection because antibody levels naturally wane over time.
Dr Helen Parry, a senior author on the study at Birmingham, said: “We’ve shown that peak antibody responses after the second Pfizer vaccination are really strongly boosted in older people when this is delayed to 11 to 12 weeks. There is a marked difference between these two schedules in terms of antibody responses we see.”
In the first weeks of the vaccine programme the UK took the bold decision to delay administering booster shots so that more elderly and vulnerable people could more quickly receive their first shots.
The move was controversial because medicines regulators approved both the Pfizer/BioNTech and Oxford/AstraZeneca vaccines on the basis of clinical trials that spaced out the doses by only three or four weeks.
Researchers from Oxford University showed in February that antibody responses were more than twice as strong when boosters of their vaccine were delayed for 12 weeks. But the latest study is the first to compare immune responses after different timings with the Pfizer/BioNTech jab.
The scientists analysed blood samples from 175 over-80s after their first vaccine and again two to three weeks after the booster. Among the participants 99 had the second shot after three weeks, while 73 waited 12 weeks. After the second dose, all had antibodies against the virus’s spike protein, but the level was 3.5 times higher in the 12-week group.
The researchers then looked at another arm of the immune system, the T cells that destroy infected cells. They found that T cell responses were weaker when the booster was delayed, but settled down to similar levels when people were tested more than three months after the first shot. Details are published in pre-print form and have yet to be peer reviewed.
“This study further supports the growing body of evidence that the approach taken in the UK of delaying that second dose has really paid off,” said Dr Gayatri Amirthalingam, consultant epidemiologist at Public Health England.
“Individuals need to really complete their second dose when it’s offered to them because it not only provides additional protection but potentially longer lasting protection against Covid-19.”
The findings come as new data from Public Health England suggested that the vaccination programme had prevented 11,700 deaths by the end of April 2021 in those aged 60 and over, and at least 33,000 hospitalisations in those aged 65 and over in the same period.
“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,” said Eleanor Riley, professor of immunology and infectious disease at the University of Edinburgh.
“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.”
