Hybrid immunity and protection against infection during the Omicron wave in Malta

By December 2021, administration of the third dose of COVID-19 vaccinations coincided with the spread of the Omicron variant in Europe. Questions had been raised on protection against infection conferred by previous vaccination and/or infection. Our study population included 252,433 participants from the COVID-19 vaccination registry in Malta. Data were then matched with the national testing database. We collected vaccination status, vaccine brand, vaccination date, infection history, and age. Using logistic regression, we examined different combinations of vaccine dose, prior infection status and time, and the odds of infection during the period when the Omicron variant was the dominant variant in Malta. Participants infected with Sars-Cov-2 prior to the Omicron wave had a significantly lower odds of being infected with the Omicron variant. Additionally, the more recent the infection and the more recent the vaccination, the lower the odds of infection. Receiving a third dose within 20 weeks of the start of the Omicron wave in Malta offered similar odds of infection as receiving a second dose within the same period. Time since vaccination was a strong determinant against infection, as was previous infection status and the number of doses taken. This finding reinforces the importance of future booster dose provision especially to vulnerable populations.

截至2021年12月，新冠疫苗第三剂接种工作与欧洲奥密克戎（Omicron）变异株的传播浪潮同步展开。当时学界与公众均提出疑问：既往疫苗接种或自然感染所提供的感染防护效果究竟如何？本研究的研究队列纳入了来自马耳他新冠疫苗接种登记系统的252433名受试者，随后将其数据与国家新冠检测数据库进行关联匹配。研究人员收集了受试者的疫苗接种状态、疫苗品牌、接种日期、感染史与年龄信息。针对马耳他奥密克戎变异株成为主流流行毒株期间的受试者感染风险比值，本研究采用逻辑回归分析方法，考察了疫苗剂次、既往感染状态及感染间隔时长等不同组合因素的影响。结果显示，在奥密克戎流行浪潮前已感染严重急性呼吸综合征冠状病毒2（SARS-CoV-2）的受试者，其再次感染奥密克戎变异株的风险比值显著更低。此外，既往感染时间与疫苗接种时间越接近当前，受试者的感染风险比值越低。在马耳他奥密克戎流行浪潮启动后的20周内接种第三剂疫苗的受试者，其感染风险比值与同期接种第二剂疫苗的受试者相当。接种后间隔时长、既往感染状态与疫苗接种剂次，均为影响受试者感染风险的重要决定因素。本研究结果进一步佐证了未来开展加强针接种的必要性，尤其应优先覆盖脆弱人群。