More than half the confirmed cases of the new Omicron variant in the UK have occurred following at least two vaccination doses, health officials have said.

A new technical briefing from the UK Health Security Agency (UKHSA) says 12 of the 22 known cases up to 30 November had been fully vaccinated.

Another two people infected had been given their first dose at least four weeks earlier.

Six were unvaccinated, with no data available on two of the cases.

However, the data does not tell us how well the vaccine protects against Omicron.

The majority of Delta cases are also in the fully-vaccinated, because more than 80% of the population over 12 has now had two jabs.

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The UKHSA has also released a risk assessment of the Omicron variant.

It gives the highest “red” alert against the virus for its theoretical ability, based on its mutations, to evade both vaccine and naturally-acquired immunity. It also warns that it could reduce the effectiveness of monoclonal antibody treatments.

It is important to note, however, that the UKHSA’s confidence level for the following bullet points is “low”.

• Amber status – Transmissibility Between Humans. “At least as transmissible as currently circulating variants. Omicron is transmitting rapidly and successfully. Increased transmissibility compared to Delta is biologically plausible.”
• Red status – Naturally Acquired Immunity. “Mutations suggestive of reduced protection from natural immunity. Based on experience with other variants, laboratory data on individual mutations, and structural modelling, the mutations present are very likely to reduce antibody binding.”
• Red status – Vaccine-Derived Immunity. “Mutations suggestive of reduced protection from vaccine derived immunity. The mutations present are likely to reduce antibody binding.”
• Red status – Therapeutics. “Mutations suggestive of reduced effectiveness of a treatment in UK clinical use. The mutations present are likely to reduce the binding of most available therapeutic monoclonal antibodies, based on structural modelling.”

The UKHSA added that PCR tests are able to distinguish Omicron from other variants, giving a “strong early signal” of UK infections.

So called S-gene drop out, which is indicative of Omicron and some other variants, has risen by 141% in the last week. In the previous 90 days it was fluctuating up and down, suggesting the new rise is linked to faster growth.

The document notes: “However, the number cannot be interpreted as a change in transmissibility or an increase in the absolute number.”

Monitoring of wastewater has found “very little evidence” of Omicron up to 21 November, but more recent data is being analysed.

In another update on Friday, the Scientific Advisory Group for Emergencies (SAGE), said Omicron may be more transmissible through the air.

Minutes from its latest meeting say Omicron “might show more airborne transmission”.

There are strong caveats, however. The “preliminary indications” are made with “low confidence”, the summary adds.

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In addition, confirmed Omicron cases in the UK are expected to rise sharply.

An update of genetic sequencing data by the COVID-19 Genomics UK Consortium, which does the bulk of the variant analysis, reveals there have been 56 cases of Omicron in England.

Currently the UKHSA puts the total in England at 29.

A source confirmed that there will be an official update to UKHSA figures “shortly” and the UK total “won’t be less” than 66.

COG-UK currently puts the number of confirmed cases in Scotland at 10, but First Minister Nicola Sturgeon has confirmed more infections, bringing the total for the nation to 29.

The UKHSA takes data from a couple of sources when compiling totals.

Your Omicron questions answered: Watch a special programme with COVID experts on Sky News at 7pm