Effectiveness of COVID-19 vaccine booster doses in adults aged 50 years and over during the Omicron period in Victoria, Australia.

BACKGROUND

Country-specific estimates of coronavirus disease 2019 (COVID-19) vaccine effectiveness (VE) are important for policy making, but analyses of COVID-19 VE in Australia have been limited to date.

METHODS

We used a modified Cox regression model to estimate, through the linkage of national and state-wide health and administrative datasets, the adjusted relative VE of three vs. two and four vs. three COVID-19 vaccine doses against hospitalisation and death due to COVID-19 among Victorians aged ≥ 50 years after the emergence of the Omicron SARS-CoV-2 variant. Analyses were conducted in two periods: 1 December 2021 to 19 June 2022 (Omicron BA.1/2 period; analyses of three vs. two doses); and 20 June 2022 to 7 November 2022 (Omicron BA.4/5 period; analyses of four vs. three doses).

RESULTS

Approximately 1.8 million people were included in analyses of three vs. two doses and approximately 1.2 million people were included in analyses of four vs. three doses. Adjusted relative VE against death 28 days after boosting with a third dose (compared to two doses) in individuals aged ≥ 65 years in the Omicron BA.1/2-dominant period reached 81.2% (95% confidence interval [95% CI]: 76.9-84.6%). There was also evidence for a relative benefit of a third dose in the Omicron BA.1/2 period against hospitalisation (adjusted relative VE 63.6% [95% CI: 60.1-66.8%] 28 days post-boosting) and for a fourth dose in the Omicron BA.4/5 period against hospitalisation and death in this age group. In contrast, estimates of relative VE in the 50-64 year age group were highly imprecise (for example, 52.4% [95% CI: -16.6-80.6%] against death 28 days after receipt of a third dose in the Omicron BA.1/2-dominant period).

CONCLUSION

These results confirm the benefits of vaccine boosters in the Omicron era for those aged ≥ 65 years, with the most notable gains evident from a third dose in late 2021 to mid-2022.