London, Dec 11 : Two-shots of a Covid vaccine may not be enough against the Omicron variant, and a third dose or a booster shot is necessary to prevent the super mutant variant, according to a UK study. The study, released by the UK Health Security Agency (UKHSA), showed that a full two-dose vaccination course was less effective against symptomatic disease with Omicron than with the original strain of Covid-19 or the Delta variant. On the other hand, a moderate to high vaccine effectiveness of 75 per cent is seen in the early period after a booster dose. "Early estimates of vaccine effectiveness (VE) against symptomatic infection find a significantly lower VE for Omicron infection compared to Delta infection. Nevertheless, a moderate to high vaccine effectiveness of 70 to 75 per cent is seen in the early period after a booster dose," the agency said in a statement. The study comes as another 448 cases of Omicron were confirmed in the UK, taking the number reported so far to 1,265. The total number of Covid cases recorded on Friday was 58,194 -- the highest figure since January 9, the BBC reported. The report analysed data from 581 Omicron cases and thousands of Delta cases to calculate how effective the vaccines were against the new variant. The analysis is based on limited data, but showed a dramatic drop in effectiveness for the Oxford-AstraZeneca vaccine and a significant drop off for two doses of Pfizer, the report said. The 75 per cent protection against Covid symptoms after a booster is not as high as against previous variants. There was not enough data to analyse the Moderna or Janssen vaccines, but there is no reason to think they would have different results. But, "it will be a few weeks before effectiveness against severe disease with Omicron can be estimated, however based on this experience, this is likely to be substantially higher than the estimates against symptomatic disease", the agency said. At the same time, the duration of restored protection after mRNA boosting is also not known at this juncture, it said. "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," Dr Mary Ramsay, head of immunisation at the UKHSA, was quoted as saying. "We expect the vaccines to show higher protection against the serious complications of Covid-19, so if you haven't yet had your first two doses please book an appointment straight away," she added. The real-world data backs up laboratory studies that showed a 40-fold reduction in the ability of antibodies from double-vaccinated people to take out the virus. There is optimism that vaccines will still keep many people out of hospital even if more do get Covid. Data on severity could be published next week. But, the UKHSA estimates that more than half of all cases in the country will be Omicron by mid-December and that if growth continues unabated there will be more than 100,000 cases a day by the end of the month. /IANS