The rest of the population is likely to also be offered another booster, but perhaps later than this, the national clinical director told Parliament’s Covid-19 recovery committee on Thursday.
Rounds of vaccination may then become annual or more frequent.
Prof Leitch was asked by the committee about plans to give a fourth dose of Covid-19 vaccine to over-60s in Israel.
In Scotland, fourth doses have so far only been offered to those who are immunocompromised, who have received three primary doses and a booster.
The Joint Committee on Vaccination and Immunisation (JCVI), which guides vaccination in Scotland, is reportedly gathering data on a possible further booster round.
Prof Leitch told the committee the decision would be kept “under review”, but is difficult to predict with certainty.
He has previously compared waning immunity to a “dimmer” light switch, emphasising that it reduces gradually rather than switching on and off abruptly.
“It's difficult to predict entirely, because it looks as though the immunity dimmer switch gets turned up quite well by the booster dose,” he said.
“Some people of course, the immunosuppressed, got three primary doses and then a booster, so we do have some people on their fourth dose inside Scotland.
“That dimmer switch has to stay up, and as soon as it begins to fade, we will know from trials, and then the joint committee [on vaccination and immunisation] will make decisions and give advice about what to do next.
“I would not at all be surprised if into next year, spring, summer, we'd be vaccinating the vulnerable again.
“I simply don’t know if we will need to vaccinate the whole population again as quickly as that. It may be a little bit later.
“Most of the smart money is probably on doing the vulnerable first and then do the rest of the population again.
“And then that might be an annual occurrence or even quicker than that, and the joint committee will decide that based on the files.”