Effectiveness of XBB.1.5 monovalent COVID-19 vaccine against COVID-19 mortality in Australians aged 65 years and older during August 2023 to February 2024
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Background
There is limited data on the effectiveness of the SARS-CoV-2 monovalent XBB.1.5 variant vaccine against COVID-19 mortality
Methods
We used Australian census data linked to the Australian Immunisation Register and death registrations to estimate COVID-19 booster effectiveness according to booster type and recency in adults aged 65+ years in Australia during August 2023-February 2024, a period dominated firstly by XBB-related SARS-CoV-2 Omicron subvariants and then the BA.2.86-related JN.1. Survival analysis, adjusted for age, sex, and other sociodemographic and health measures, was used to estimate vaccine effectiveness.
Results
We followed 4.12 million adults aged 65+ years from 1 August 2023 to 29 February 2024. By 29 February, 581146 doses of the XBB.1.5 vaccine were administered, and 1620 COVID-19-specific deaths occurred. COVID-19 mortality rates were 72/100000 person-years in people who received a COVID-19 booster >365 days earlier, and 21/100000 in those who received the XBB.1.5 booster in the last 3 months. The relative vaccine effectiveness (rVE) of XBB.1.5 booster receipt in the last 3 months against COVID-19 mortality was 74.7% (95%CI 59.9-84.1%). The rVE for those receiving other booster types in the last 3 months was 51.6% (39.3-61.4%). Booster rVE against COVID-19 mortality waned. Compared to those who received a COVID-19 booster >365 days earlier, rVE for a booster within 3-6 months earlier was 31.2% (18.9-41.6%) and for a booster received 6-12 months earlier rVE was 13.1% (1.8-23.2%). rVE estimates were similar in analyses restricted to 1 December 2023 to 29 February 2024 when the dominant Omicron subvariant was JN.1.
Conclusions
Recent booster vaccination with the XBB.1.5 monovalent COVID-19 vaccine is highly effective in preventing COVID-19 mortality including in the period when the JN.1 subvariant circulated, supporting recommendations for 6-monthly boosting in older adults. Evaluation of vaccination effectiveness against other health outcomes, such as COVID-19 hospitalisations and ICU admission, would help further document vaccination benefits.
