The effectiveness of the booster jab could wane off after just ten weeks, the latest data suggests.
The UK Health Security Agency last night published a report into the risk of the spread of the Omicron variant of coronavirus.
As of December 20, more than 56,000 cases of Omicron have been identified in England, the report said.
In total, 132 people have been admitted to hospital with Omicron of which 40 per cent were in London.
Of those patients, 17 had received a booster dose, 74 a second dose and 27 were not vaccinated.
Fourteen people are reported to have died within 28 days of an Omicron diagnosis with their ages ranging from 52 to 96-years-old.
The positive news is that the early data confirms that Omicron appears to be milder than the Delta variant, with risk of hospitalisation reduced by up to 70 per cent.
However, it also shows lower vaccine effectiveness.
"There is evidence of waning of protection against symptomatic disease with increasing time after dose 2, and by 10 weeks after the booster dose, with a 15 to 25 per cent reduction in vaccine effectiveness after 10 weeks," the report says.
"This waning is faster for Omicron than for Delta infections.
"There are insufficient severe cases of Omicron as yet to analyse vaccine effectiveness against hospitalisation, but this is expected to be better sustained, for both primary and booster doses."
This morning, UK Health Security Agency chief Jenny Harries said the findings about milder disease offer a "glimmer of Christmas hope".
But she warned that more information is needed, particularly about the impact on elderly and more vulnerable patients.
Dr Harries told BBC Radio 4's Today programme: "There is a glimmer of Christmas hope in the findings that we published yesterday, but it definitely isn't yet at the point where we could downgrade that serious threat."
The rapid spread of Omicron has seen it become the "dominant strain now right across the UK".
Dr Harries added: "What we have got now is a really fine balance between something that looks like a lower risk of hospitalisation - which is great news - but equally a highly transmissible variant and one that we know evades some of our immune defences, so it is a very balanced position."
