The Effectiveness of COVID-19 Vaccination in a Regional Population

Background: Research on vaccine effectiveness is crucial to guide public health policy. In an Australian regional area, we evaluated vaccine effectiveness and waning immunity for the prevention of hospitalizations associated with SARS-CoV-2.Methods: COVID-19 notification data between January-2022 and March-2022 for the Central Queensland region were linked to the Australian Immunization Register and hospital administration data. Vaccinated and unvaccinated cohorts were compared to estimate VE for their risk of hospitalization caused by the Omicron variants. VE was calculated after primary immunization with two doses of BNT162b2, ChAdOx1-nCoV-19, or mRNA-1273 vaccine and after a booster dose. Findings: Out of 9,682 persons studied, 7,244 (75%) were vaccinated at the time of infection. Only 47 (0.5%) cases were hospitalized and 4 (0.04%) required ICU admission, with no deaths observed. VE for symptomatic hospital admission for persons with only a primary course (of either BNT162b2, ChAdOx1 or mRNA-1273) was 70.2% (95%CI 45.2, 84.4; p<0.001), and for persons with a booster was 82.1% (95%CI 48.3, 95.8; p=0.005). In First Nations population (n=665), VE for symptomatic hospital admission was 69.7% (95%CI -57.9, 95.8; p=0.172).Interpretations: The symptomatic hospitalization rate for Omicron variant of SARS-CoV-2 was low. VE against symptomatic hospitalization in SARS-CoV-2 infections was high in persons with booster doses. Further work on understanding waning immunity is important for optimal booster dose scheduling.Funding Information: This work was supported by the Royal Australasian College of Physicians 2023 Research Development Grant - 2023RDG00006, Moderna Global Fellowship 2022-01 and the Queensland Advancing Clinical Research Fellowship (PJ-70405-A034-X000-HE2993).Declaration of Interests: All authors but NRS declare no conflict of interests. NRS was awarded the Moderna Global Fellowship 2022- 01.Ethical Approval: This study was exempted from formal ethical review by the Townsville Hospital and Health Service Human Research Ethics Committee (HREC/82361).