UK Data on Vaccine Efficacy against the Omicron Variant

December 27, 2021

A UK Health Security Agency technical briefing has reported that the new Omicron variant is able to evade antibodies from both vaccination and previous infection. Vaccine effectiveness was reportedly lower for Omicron than the Delta variant, with evidence of waning protection against symptomatic disease by 10 weeks after a booster dose. Using data up until December 20, 2021, 132 individuals with laboratory confirmed cases of Omicron were hospitalized. 17 had received a booster dose, 74 had received a second dose to a primary series, and 27 were not vaccinated. Less than 10 patients’ vaccine statuses were unaccounted for or had only received one dose.

Vaccines included for analysis were AstraZeneca, Pfizer and Moderna. Regardless of mixing-and-matching, “effectiveness was lower for Omicron compared to Delta.” It was assessed that an AstraZeneca primary series followed by an mRNA booster dose was 60% effective at preventing symptomatic infection 2-4 weeks after the booster dose. When given a Pfizer booster, efficacy decreased by the 10 week mark to 35%; vaccine effectiveness decreased to 45% when given a Moderna booster. “Among those who received a Pfizer primary course, vaccine effectiveness was around 70% after a Pfizer booster, dropping to 45% after 10-plus weeks and stayed around 70 to 75% after a Moderna booster up to 9 weeks after booster.”

Compared to Delta, Omicron is “currently more concentrated in young adult age groups (20 to 29) and less prevalent in children… [Moreover,] the risk of hospital admission for a person detected as a case of Omicron appears reduced compared to a case of Delta.” In fact, the risk of emergency care or hospital admission with the Omicron variant was assessed to be 3/5 of that for Delta. That being said, these figures are based on preliminary analysis using data from small cohorts of emergency department and hospital admissions of patients infected with the Omicron variant.





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