LONDON: Booster COVID-19 shots significantly restore protection against mild disease caused by the Omicron variant, in part reversing an otherwise steep drop in vaccine effectiveness, the UK Health Security Agency said on Friday.

The early findings from a real-world analysis are some of the earliest data on the protection against Omicron outside of lab studies, which have shown reduced neutralising activity against Omicron.

"These early estimates should be treated with caution but they indicate that a few months after the second jab, there is a greater risk of catching the Omicron variant compared to Delta strain," said Dr Mary Ramsay, Head of Immunisation at the UKHSA, adding that protection against severe disease was expected to remain higher.

"The data suggest this risk is significantly reduced following a booster vaccine, so I urge everyone to take up their booster when eligible."

In an analysis of 581 people with confirmed Omicron, two doses of AstraZeneca or Pfizer-BioNTech vaccines provided much lower levels of protection against symptomatic infection compared with what they provide against Delta.

However, when boosted with a dose of Pfizer vaccine, there was around 70% protection against symptomatic infection for people who initially received AstraZeneca, and around 75% protection for those who received Pfizer.

That compares with estimated protection against infection from Delta following a booster of around 90%.

UKHSA reiterated it found that Omicron had a growth advantage over Delta, and a 3-to-8 fold increased risk of reinfection.

It said two UK studies which have yet to be presented publicly and three international studies suggested Omicron gave a 20 to 40-fold reduction in neutralising antibodies compared with the viruses used to develop vaccines.

UKHSA said that while no cases of Omicron had yet resulted in hospitalisation or death, there was insufficient data to assess the severity of Omicron.

At current growth rates, Omicron would account for more that 50% of all COVID-19 infections by mid-December, UKHSA said, with Britain exceeding one million infections by the end of the month, as new measures come into force in England to slow the spread of Omicron.

"Rising cases of the Omicron variant coupled with the new data today should be a wake-up call for those who haven’t yet had their booster or, indeed, any vaccine," National Health Service medical director Stephen Powis said.