This suggests that the protection against re-infection with Omicron of a past infection may be as low as 19%.

The study also found no evidence that Omicron had a lower severity than Delta, but the data on hospital admissions was very low at the time of the study.

According to the data, boosters are essential for controlling Omicron, but they can lose effectiveness over time.

Researchers estimated that the proportion of Omicron among all Covid cases between November 29 and December 11 doubled every other day until December 11.

Based on this, they also estimate that the reproduction number (R) of Omicron was greater than 3, during the study period.

Professor Neil Ferguson of Imperial College London said: “This study provides further evidence of the very important extent to which Omicron can evade previous immunity, given both by infection or by vaccination.

“This level of immune evasion means that Omicron poses a major and imminent threat to public health.”

There is a significantly increased risk of developing a symptomatic Omicron case compared to Delta for those who have had two or more weeks after their second vaccine dose and two or more weeks after their booster dose (for AstraZeneca and Pfizer vaccines), according to the report.

Based on estimates of vaccine efficacy against symptomatic Delta variant infection, this translates to vaccine efficacy estimates against symptomatic Omicron infection between 0% and 20% after two doses, and between 55% and 80% after a booster dose.

Scientists used data from the British Health Security Agency (UKHSA) and the NHS for all PCR-confirmed Covid cases in England who passed a test between November 29 and December 11 of this year.

Professor Azra Ghani, Imperial College London, said: “Given the rapid spread of the Omicron variant to date, it is now very likely that it will replace the Delta variant circulating around the world in the near future. weeks to come.

“The emerging immunogenicity data clearly indicate substantial reductions in neutralizing antibodies, while preliminary estimates of vaccine efficacy demonstrate a substantial reduction in protection against mild disease.

“Our estimates suggest that this is likely to result in small but significant reductions in effectiveness against serious illness and death.

“A remaining uncertainty is the severity of the disease caused by the Omicron variant compared to the disease caused by the previous variants.

“While it may take several weeks to fully understand this, governments will need to put plans in place now to mitigate any potential impact.

“Our results demonstrate the importance of administering booster doses as part of a broader public health response.

“Prioritizing these boosters for high-risk populations over primary vaccination in younger age groups should be part of this response in countries where dose supply is limited.” “

A separate report from researchers at Imperial looked at the effectiveness of the booster doses against Omicron.

After two initial doses and a booster vaccine with an mRNA vaccine, Pfizer BioNTech or Moderna, neutralizing antibodies (which prevent infection) were estimated to be 1.6 times of the levels after the second dose of Pfizer vaccine. BioNTech, and 3.3 times their levels after the second dose of the Oxford-AstraZeneca vaccine.

However, the researchers estimate that the levels of antibodies produced against Omicron could be 4.5 times lower than those produced against the Delta variant.

They also found that this reduction in neutralizing antibodies could affect the effectiveness of the vaccine against severe disease.

In the worst-case scenario, where the rate of antibody decay after a booster dose is the same as seen after the first two doses, researchers predict that the vaccine’s effectiveness against severe disease could drop by 96.5%. against Delta at 80.1% against Omicron, 60 days after the primary series followed by a booster of Pfizer vaccine.

If the decomposition rate is half that rate, the drop is estimated to be 97.6% vs. Delta to 85.9% vs. Omicron.

However, the researchers say that while these numbers are currently associated with a high degree of uncertainty, they indicate that Omicron variant-specific vaccines and / or additional boosters will likely be needed to restore protection.

According to the report, the distribution of Omicron by age, region and ethnicity currently differs from that of Delta, with 18-29 year olds, London area residents and those of African descent having significantly Omicron infection rates. higher compared to Delta.

The modeling made a number of estimates of what infection rates and deaths might look like under various scenarios.

Among them, one scenario indicates that there could be as many as 100 daily deaths per million in a country where the majority of people over the age of 10 have been vaccinated and the majority of those over 40 have received boosters.

However, Prof Ghani said: “I think this is an illustration of the need for action, rather than a prediction.”

Professor Ferguson said: “We are ahead of the rest of Europe in terms of the Omicron wave.

“I recognize that time is running out. I think if we are going to make any further decisions, which remains to be seen, they will probably have to be taken in the next week or two to have a substantial impact. “