Effectiveness of COVID-19 vaccines against Omicron and Delta hospitalisation: test negative case-control study
Abstract
Background
The omicron (B.1.1.529) variant has been associated with reduced vaccine effectiveness (VE) against infection and mild disease with rapid waning, even after a third dose, nevertheless omicron has also been associated with milder disease than previous variants. With previous variants protection against severe disease has been substantially higher than protection against infection.
Methods
We used a test-negative case–control design to estimate VE against hospitalisation with the omicron and delta variants using community and in hospital testing linked to hospital records. As a milder disease, there may be an increasing proportion of hospitalised individuals with Omicron as an incidental finding. We therefore investigated the impact of using more specific and more severe hospitalisation indicators on VE.
Results
Among 18-64 year olds using all Covid-19 cases admitted via emergency care VE after a booster peaked at 82.4% and dropped to 53.6% by 15+ weeks after the booster; using all admissions for >= 2 days stay with a respiratory code in the primary diagnostic field VE ranged from 90.9% down to 67.4%; further restricting to those on oxygen/ventilated/on intensive care VE ranged from 97.1% down to 75.9%. Among 65+ year olds the equivalent VE estimates were 92.4% down to 76.9%; 91.3% down to 85.3% and 95.8% down to 86.8%.
Conclusions
With generally milder disease seen with Omicron, in particular in younger adults, contamination of hospitalisations with incidental cases is likely to reduce VE estimates against hospitalisation. VE estimates improve and waning and waning is more limited when definitions of hospitalisation that are more specific to severe respiratory disease are used.
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