Effectiveness of COVID-19 vaccines against hospitalisation, death and infection over time in Aotearoa New Zealand: a retrospective cohort study.

Aims: This study aimed to evaluate the effectiveness of COVID-19 vaccines in preventing COVID-19 outcomes when the Omicron variant was predominant in Aotearoa New Zealand.

Methods: We conducted a retrospective cohort study using routinely available data (8 December 2020-28 February 2023). We evaluated the vaccine effectiveness (VE) of COVID-19 vaccines using the Cox proportional-hazards model, adjusting for covariates.

Results: The VE against COVID-19 hospitalisation (VEH) for the second booster dose compared to no vaccination was found to be 81.8% (95% confidence interval [95% CI]: 73.6-87.5) after 1 month post-vaccination. After 4 months, VEH was 72.2% (95% CI: 58.5-81.4), and after 6 months VEH was 49.0% (95% CI: 7.9-71.8). Similarly, VEH decreased after the first booster dose (1-month VEH=81.6% [95% CI: 75.6-86.1]; 2 months VEH=74.7% [95% CI: 68.2-79.9]; and 6 months VEH=57.4% [95% CI: 45.8-66.6]). VE against COVID-19 death (VED) was 92.9% (95% CI: 82.1-97.2) 2 months after the first booster vaccination, with VED being sustained until months 5 and 6 (VED=87.2%; 95% CI: 67.4-94.9). The VE after the second dose of the vaccine against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection (VEI) (real-time polymerase chain reaction [RT-PCR]) was sustained at 5 months post-vaccination (40.6%; 95% CI: 25.6-52.5).

Conclusion: We provide a comprehensive quantification of both VE and VE waning. These findings can guide policymakers to help evaluate the COVID-19 vaccination programme and minimise the effect of future COVID-19 in Aotearoa New Zealand.