The Omicron variant has been associated with reduced vaccine effectiveness (VE) against mild disease with rapid waning. Meanwhile Omicron has also been associated with milder disease. Protection against severe disease has been substantially higher than protection against infection with previous variants. We used a test-negative case-control design to estimate VE against hospitalisation with the Omicron and Delta variants using PCR testing linked to hospital records. We investigated the impact of increasing the specificity and severity of hospitalisation definitions on VE. Among 18–64-year-olds using cases admitted via emergency care, VE after a 3rd dose peaked at 82.4% and dropped to 53.6% by 15+ weeks after the 3rd dose; using all admissions for > = 2 days stay with a respiratory code in the primary diagnostic field VE ranged from 90.9% to 67.4%; further restricting to those on oxygen/ventilated/intensive care VE ranged from 97.1% to 75.9%. Among 65+ year olds the equivalent VE estimates were 92.4% to 76.9%; 91.3% to 85.3% and 95.8% to 86.8%. Here we show that with milder Omicron disease contamination of hospitalisations with incidental cases is likely to reduce VE estimates. VE estimates increase, and waning is reduced, when specific hospitalisation definitions are used.

Omicron 变体与针对轻度疾病的疫苗有效性 (VE) 降低有关，并且会迅速减弱。同时，Omicron 也与较轻的疾病有关。对严重疾病的保护远远高于对先前变体感染的保护。我们使用测试阴性病例对照设计，使用与医院记录相关的 PCR 检测来估计 VE 与 Omicron 和 Delta 变体的住院率。我们调查了增加住院定义的特异性和严重性对 VE 的影响。在使用通过急诊入院的病例的 18-64 岁人群中，VE 在第 3 次给药后达到峰值 82.4%，并在第 3 次给药后 15+ 周下降至 53.6%；使用所有入院时间 > = 2 天并在主要诊断字段中使用呼吸代码 VE 的范围为 90.9% 至 67。4%；进一步限制在氧气/通风/重症监护的患者 VE 范围为 97.1% 至 75.9%。在 65 岁以上的人群中，等效 VE 估计值为 92.4% 至 76.9%；91.3% 至 85.3% 和 95.8% 至 86.8%。在这里，我们表明，对于较轻的 Omicron 疾病，偶发病例住院的污染可能会降低 VE 估计值。当使用特定的住院定义时，VE 估计会增加，而减弱会减少。