A fourth dose of the Covid-19 vaccine will “probably” be needed over the next few years, Scotland’s national clinical director has said.

Scots have been asked to get a third dose of the jab in a bid to stifle the new Omicron variant, with figures released on Thursday showing more than 70% of the population who will be eligible by the end of the year already having done so.

But Professor Jason Leitch has said there may be a need for at least some fourth doses to be administered.

Speaking on BBC Radio Scotland’s Good Morning Scotland programme on Friday, Prof Leitch said: “It would seem that we will probably need some kind of timed booster or next dose over the next few years.

“We don’t know that for sure – it may be that we just offer that to the vulnerable, those who are maybe a bit older.”

He said immunity was “like a dimmer switch, not a light switch”, adding: “If you can turn the dimmer switch up and keep it up, then that’s what you want to do, because this disease is at its worst when it gets people without immunity.”

The immunity of some in the booster programme, which has been rolled out to all adults, will be monitored, Prof Leitch said, with the Joint Committee on Vaccination and Immunisation (JCVI) tasked with advising whether another dose is required.

Meanwhile, he said he would advise that the vaccine passport scheme should be extended to include more venues than it currently does.

A certification app was put in place in early October, which would provide proof of double vaccination in order to enter large events or nightclubs.

Following a question from Glasgow nightclub owner Donald MacLeod about whether the scheme should be scrapped, Prof Leitch said: “I think he’s right that we need to look at Covid certification in light of the new variant.

“I think, though, the public health advice would actually be probably to expand it to allow more places to open in light of the work around the new variant.

“Donald won’t like that, he’ll be yelling at the radio, but that would probably be the public health position.

“The final decision about that will be made by the First Minister of Wales for theirs and the First Minister of Scotland for ours.”

Prof Leitch also stressed that the spike in cases was caused by the Omicron variant, not by a failure of the Covid passport.

The Scottish Government considered the extension of the scheme last month, but ministers decided against the move because, at the time, cases were “currently stable and indeed slightly declining” according to First Minister Nicola Sturgeon, before the new variant took hold.