Third vaccine dose boosts Omicron protection, with some waning

Two doses of the AstraZeneca/Oxford, Pfizer/BioNTech, or Moderna COVID-19 vaccine offered limited protection against symptomatic infection with the SARS-CoV-2 Omicron variant, and although a third dose with any of the three vaccines boosted effectiveness substantially, it waned a bit over time.These findings, from a study by UK Health Security Agency researchers, were published yesterday in the New England Journal of Medicine.The researchers used National Immunization Management System data to estimate vaccine effectiveness (VE) against symptomatic illness caused by infection with the Omicron and Delta variants from Nov 27, 2021, to Jan 12, 2022.

They compared the odds of vaccination in symptomatic residents who tested positive for COVID-19 with those who tested negative.Moderna boosters outperform PfizerA total of 886,774 residents of England were diagnosed as having COVID-19 infections caused by Omicron, while 204,154 were infected with Delta, and 1,572,621 were included in the study as test-negative controls.Throughout the study, VE was higher against Delta than Omicron for all combinations of the primary vaccine series and booster doses.

Two doses of the AstraZeneca vaccine became ineffective after 20 weeks, while two doses of the Pfizer vaccine were 65.5% effective (95% confidence interval [CI], 63.9% to 67.0%) at 2 to 4 weeks, falling to 8.8% (95% CI, 7.0% to 10.5%) at 25 weeks.Of recipients of a primary course of the AstraZeneca vaccine, a Pfizer booster dose increased VE to 62.4% (95% CI, 61.8% to 63.0%) by 2 to 4 weeks, dropping to 39.6% (95% CI, 38.0% to 41.1%) by 10 weeks.

VE among Pfizer primary course recipients climbed to 67.2% (95% CI, 66.5% to 67.8%) after a Pfizer booster and then decreased to 45.7% (95%