A study of 500 healthcare workers found a 10-week interval between doses produces higher antibody levels, as well as a higher proportion of infection-fighting T cells than a four-week gap.
Oxford University Professor Susanna Dunachie joint chief investigator in the Pitch study, said the decision to opt for eight weeks was a “balance” of wider issues.
This dose gap is a “sweet spot” currently, she said, as it balances higher protection with a longer gap against getting the second dose as soon as possible.
Health secretary Humza Yousaf labelled the study “further evidence” that the Scottish Government is right to follow the JCVI advice of an eight week gap, despite repeated calls from Labour to shorten this.
He said: “Scottish Labour wanted us to recklessly abandon expert advice from JCVI. They should hold their hands up and admit they got this one badly wrong.”
Labour health spokesperson Jackie Baillie renewed the call for the gap to be cut, saying Mr Yousaf’s response was “simplistic”.
She said: “The researchers make clear that even if eight weeks is the optimum time-frame in theory, in reality there is a balance to strike. We cannot ignore the urgent need to give people as much protection as possible as the Delta variant continues to infect too many people in Scotland.
“These are not easy calls to make and they require serious thought – but the SNP are more concerned with pathetic attempts at political point-scoring than genuinely weighing up the range of evidence available.”
For the study, researchers recruited 503 healthcare workers, 44 per cent (223) of whom previously had Covid-19, and studied the immune responses generated by the Pfizer jab.
They found that both short (three to four-week) and long (10-week) dosing intervals of the Pfizer vaccine generated strong antibody and T cell immune responses.
But the longer schedule led to higher antibody levels and a higher proportion of helper T cells, which according to the researchers, supports immune memory.
The scientists found that after the second dose, a wider gap also resulted in higher neutralising antibody levels against the Delta variant and all other variants of concern.