SARS-CoV-2 Vaccine Effectiveness against Omicron Variant in Infection-Naive Population, Australia, 2022

Abstract

SARS-CoV-2 transmission in Western Australia, Australia, was negligible until a wave of Omicron variant infections emerged in February 2022, when >90% of adults had been vaccinated. This unique pandemic enabled assessment of SARS-CoV-2 vaccine effectiveness (VE) without potential interference from background immunity from prior infection. We matched 188,950 persons who had a positive PCR test result during February-May 2022 to negative controls by age, week of test, and other possible confounders. Overall, 3-dose VE was 42.0% against infection and 81.7% against hospitalization or death. A primary series of 2 viral-vectored vaccines followed by an mRNA booster provided significantly longer protection against infection >60 days after vaccination than a 3-dose series of mRNA vaccine. In a population free from non-vaccine-derived background immunity, vaccines against the ancestral spike protein were ≈80% effective for preventing serious outcomes from infection with the SARS-CoV-2 Omicron variant.

Keywords: 2019 novel coronavirus disease; Australia; COVID-19; Omicron; SARS-CoV-2; SARS-CoV-2–naive; coronavirus disease; respiratory infections; severe acute respiratory syndrome coronavirus 2; vaccine effectiveness; viruses; zoonoses.

Figure 1

Number of SARS-CoV-2–positive test results reported by day, by test type, Western Australia, Australia, February 1–June 30, 2022. Source: COVID-19 Public Health Operations, WA Health (D. Barth, COVID-19 Public Health Operations, WA, Australia, pers. comm., email, 2022 Dec 1).

Figure 2

Weekly proportion of assigned lineages for sequenced SARS-CoV-2 specimens, by week of sample collection, Western Australia, Australia, February 1–May 31, 2022. Source: PathWest Laboratory Medicine (C. Sikazwe, Pathwest Laboratory, WA, Australia, pers. comm., email, 2022 July 15).

Figure 3

Selection of SARS-CoV-2 cases positive by PCR and of negative controls for analysis of vaccine effectiveness, Western Australia, Australia, February 1–May 31, 2022. IRSAD, Index of Relative Socioeconomic Advantage and Disadvantage.

Figure 4

Vaccine effectiveness against breakthrough infection of any severity, by time since first booster dose versus unvaccinated controls, Western Australia, Australia, February 1–May 31, 2022. Error bars indicate 95% CIs.

Figure 5

Vaccine effectiveness against breakthrough infection of any severity, by time since booster vaccination, for homologous (all mRNA vaccines) versus heterologous (ChAdOx1 primary, mRNA booster) vaccination series, Western Australia, Australia, February 1–May 31, 2022. Error bars indicate 95% CIs.