The finding comes days after the UK government reduced the gap between the two doses of COVID-19 vaccines to eight weeks, while India extended the interval between two doses of the Covishield preventive to 12-16 weeks, up from the previous maximum of eight weeks.
The study conducted on 175 people aged over 80 is the first direct comparison of the immune response in any age group between those who are given the second Pfizer vaccine dose at a three-week interval and those at a 12-week interval.
The Pfizer vaccine was originally authorised for a three-week interval between doses.
Several countries, including the UK, chose to expand this to a 12-week interval to allow a higher percentage of the population to receive one vaccine dose quicker.
However, the UK last week cut the gap from 12 weeks to eight weeks in view of the spread of the B.1.617 variant that originated in India.
The yet-to-be peer-reviewed research found that extending the second dose interval to 12 weeks increased the peak SARS-CoV-2 spike specific antibody response 3.5-fold compared to those who had the second vaccine at three weeks.
The team concluded that extending administration of the second Pfizer vaccine to 12 weeks potentially enhances and extends antibody immunity, which is believed to be important in virus neutralisation and prevention of infection.
“SARS-CoV-2 vaccines have been remarkably effective in providing large-scale protection against infection and symptomatic disease – but many questions remain regarding their optimal delivery for provision of effective and sustained immunity,” said study first author Helen Parry, from the University of Birmingham, UK.
“The study is crucial, particularly in older people, as immune responses to vaccination deteriorate with age,” said Parry.
Understanding how to optimise COVID-19 vaccine schedules and maximise immune responses within this age group is vitally important, the researcher said.
“The enhanced antibody responses seen after an extended interval may help to sustain immunity against COVID-19 over the longer term and further improve the clinical efficacy of this powerful vaccine platform,” said corresponding author of the study Paul Moss, from the University of Birmingham.
“Our research findings may be important in the development of global vaccination strategy as extension of interval of the second vaccine dose in older people may potentially reduce the need for subsequent booster vaccines,” Moss said.
The research saw the team taking blood samples for analysis in the lab after participants’ first vaccine and then again two to three weeks after participants had received their second vaccine.
Of the cohort, 99 participants had the second vaccine at three weeks, while 73 had the second dose at 12 weeks.
Participants who had previous infection — 10 in the three-week interval group and five in the 12-week interval group — were excluded from the analysis as previous infection has been shown to have a major impact on the immune response to vaccination.
After their second vaccine, spike protein-specific antibodies were detected in all participants no matter how far apart their doses were.
Spike protein helps the virus to infect human cells.