This week, the Government extended the booster campaign to all adults and is shortening the gap to just three months in an attempt to give more people protection should omicron cases start to surge.

On Friday, the UK Health Security Agency published its first variant technical briefing about omicron, but said that the delta variant was still making up more than 99 per cent of cases.

Although 12 of the first 22 cases were among people who were double-jabbed, experts cautioned that the numbers were small and reflected the background vaccination coverage in the population.

So far, nobody with omicron in England has died or been hospitilised. Jenny Harries, the chief executive of the UK Health Security Agency, said: “Thanks to very high levels of vaccine coverage we already have a robust wall of defence against Covid-19 as new variants emerge.

“We are working as fast as possible to gather more evidence about any impact the new variant may have on the severity of disease or vaccine effectiveness.

“Until we have this evidence, we must exercise the highest level of caution in drawing conclusions about any significant risks to people’s health.

“The most important thing everyone can do now is to get any vaccine dose that you are eligible for – it is by far the most effective action you can take to protect yourself, your families and your communities.”