But the National Clinical Director added that the Scottish Government target of offering a first dose to all over-18s by the end of July should still be hit.
It comes after the Joint Committee on Vaccination and Immunisation (JCVI) advised that those aged 18 to 29 should be offered an alternative to the AstraZeneca vaccine where possible over a potential link to “extremely rare” blood clots.
This has caused the Scottish Government to consider its vaccination plan for people both under and over-30.
Prof Leitch said vaccination of under-30s may take longer than expected, as they will need to be given Pfizer or Moderna vaccines which have more demanding storage requirements, and that the programme for over-30s may be tweaked slightly to give more of the AstraZeneca supply to them.
“We’re not doing first-dose non-clinically vulnerable 30-year-olds, and therefore we have time to work out what Pfizer and Moderna will mean,” Prof Leitch told a media briefing on Thursday.
“I imagine that when we go to that stage, we may be asking them to travel a little bit more because Pfizer and Moderna are freezer vaccines, not fridge vaccines, and therefore, the logistics of it are slightly more difficult, and that might slow us somewhere in those middle weeks.
“But I actually believe we can get to July on time, and the pace will be exactly as the supply allows it to be.
“It may mean we need to do things, exactly as you would expect, we might have to stop Moderna for “x” group, and give them AstraZeneca, as we as we think about what we do with the others.”
He added: “If we move AstraZeneca into the over-50s, like me, it means we take Moderna and Pfizer out of the over-50s and reserve it for the under-30s.
"The total remains the same, what we’re doing is redistributing inside that total.”
The Scottish Government said it would look at any potential impact of the JCVI decision on the vaccination programme.
"We are currently considering whether there may be any implications for the timescales for our national vaccination programme but at this stage we remain on track to offer first injections to all adults by the end of July,” said Deputy Chief Medical Officer Nicola Steedman on Wednesday.
The MHRA’s chief executive, Dr June Raine, said there is a “reasonably plausible” link between the AstraZeneca jab and rare blood clots, but added that they are “extremely rare”.
In all age groups above 30, any slight risk from the vaccine is outweighed by the much larger risk from the virus.
Up to March 31, the MHRA received 79 reports of blood clots accompanied by low blood platelet count, all in people who had their first dose of the vaccine, out of around 20 million doses given.
Prof Leitch said due to patient confidentiality it is not known how many of these people were in Scotland.
Of these 79, a total of 19 people have died, three of whom were under 30, although it has not been established what the cause was in every case.