While JCVI advice is not binding in devolved nations, Nicola Sturgeon has previously indicated the Scottish Government will comply with this.
The JCVI has recommended that all adults aged 18 to 39 should be offered a booster dose in descending age order, with priority also given to more vulnerable individuals. Those aged 40 and over are already eligible for a booster.
The committee has also reduced the amount of waiting time recommended between the second and third doses to three months.
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In further advice, young people aged 12 to 15 should be offered a second dose of the Pfizer vaccine, no sooner than 12 weeks after their first dose.
In a briefing on the decision, deputy chief medical officer of England Professor Jonathan Van-Tam urged people not to panic, but not to ignore the “weather forecast” coming from South Africa.
The number of mutations present in the Omicron variant “makes us worry about a possible effect on vaccine effectiveness”, he said.
But he added: “I do not want people to panic at this stage.
“If vaccine effectiveness is reduced – as seems pretty likely, to some extent – the biggest effects are likely to be in preventing infections and, hopefully, there will be smaller effects in preventing severe disease.”
While there is little known about the new variant yet, Prof Van-Tam said vaccine boosting is the easiest thing to do while waiting for scientists to gather more information about Omicron.
Both the Moderna and Pfizer vaccines can be given as a booster for adults, the JCVI said.
Professor Wei Shen Lim, chair of the JCVI said: “Having a booster dose of the vaccine will help to increase our level of protection against the Omicron variant.
“This is an important way for us to reduce the impact of this variant on our lives, especially in the coming months.
“If you are eligible for a booster, please take up the offer and keep yourself protected as we head into winter.”
Scotland is ready to scale up the booster vaccine programme if needed, Ms Sturgeon told a briefing earlier on Monday.
“Over the weekend we have already been starting to consider the operational aspects of that,” she said.
“We are already going very, very fast in the programme, we’re the most vaccinated part of the UK. We are already trying to step that up."
Dr June Raine, chief executive of the Medicines and Healthcare products Regulatory Agency (MHRA), said: “The public can be confident that our robust regulatory assessment supports the JCVI’s recommended extension to the vaccination campaign.
“This further strengthens our ability to ensure people are protected against Covid-19 and saves lives.
“Our safety monitoring to date shows that Covid-19 vaccines continue to have a positive safety profile for the majority of people.
“When you are called for your booster dose, you can come forward confident that the benefits in preventing serious Covid-19 far outweigh any risks.”