The First Minister faced pressure from opposition MSPs to speed up Scotland’s vaccine rollout, as weekly doses fell to their lowest in four months last week.
Some 184,320 doses were given, the lowest figure since the second week in March.
Ms Sturgeon defended the rollout, and said supplies are “healthy” but second doses are constrained by needing to wait eight weeks after a first dose.
Labour leader Anas Sarwar called on the Scottish Government to reduce the gap between doses – a move the First Minister labelled “unthinkable”.
Mr Sarwar said: “The WHO advice is to administer the second dose of the vaccine after three to four weeks, the manufacturer's advice is to administer the vaccine after three to four weeks, and many countries across the world are administering the second dose of the vaccine after four weeks, and we know from the data that the significant protection you get to the Delta variant comes from the second dose of the vaccine.
“So will the government now move to a four week gap between vaccines, as recommended by the WHO, as recommended by manufacturers, and as has been led by other countries around the world?”
But Ms Sturgeon called Mr Sarwar’s comments “irresponsible”.
“In the entire lifetime of this parliament, no devolved government has gone against the recommendations of the JCVI on vaccination,” she said.
“The JCVI advice is that to maximise the effectiveness of the vaccine and the longevity of the protection of the vaccine, the dosage interval should be eight weeks between the first and the second dose.”
It would be “unthinkable” to go against JCVI advice, Ms Sturgeon said, but the Scottish Government will reduce the gap if JCVI advice changes.
Ms Sturgeon has said all adults in Scotland will be given a first vaccine dose by July 18.
But with less than a week to go, almost 450,000 Scots under 40 have not received a vaccine.
Daily first doses have declined recently, from around 20,000 per day at the beginning of June, to around 15,000 by late June, to an average of below 10,000 a day last week.