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Relative Effectiveness of COVID-19 Vaccination with 3 Compared to 2 Doses Against SARS-CoV-2 B.1.1.529 (Omicron) Among an Australian Population with Low Prior Rates of SARS-CoV-2 Infection

20 Pages Posted: 21 Jun 2022

See all articles by Bette Liu

Bette Liu

Children’s Hospital at Westmead - The National Centre for Immunisation Research and Surveillance (NCIRS)

Heather Gidding

National Centre for Immunisation Research and Surveillance

Sandrine Stepien

National Centre for Immunisation Research and Surveillance

Michelle Cretikos

NSW Ministry of Health - Population and Public Health Division

Kristine Macartney

Children’s Hospital at Westmead - The National Centre for Immunisation Research and Surveillance (NCIRS)

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Abstract

Background: Most COVID-19 vaccine effectiveness studies are in settings with significant prior community transmission of SARS-CoV-2. We estimate effectiveness of 3 versus 2 vaccine doses against SARS-CoV-2 B.1.1.529 Omicron in a mostly infection-naiive but highly vaccinated Australian population.

Methods: Cohort study of adults aged 40+ years resident in Sydney followed from 1 Jan 2022 for SARS-CoV-2 infection and COVID-19 hospitalisation or death using linked immunisation, disease notification and hospitalisation registers. Adjusted hazard ratios (aHR) and corresponding relative vaccine effectiveness (rVE) were estimated comparing 3 to 2 vaccine dose recipients by time since dose receipt, vaccine brand, and prior infection. Absolute risk reductions and numbers needed to boost by age groups were calculated.

Findings: 2,053,123 infection-naiive individuals (mean age 59 years) were followed for 327,272 person-years for infection and 224,269 person-years for severe outcomes (hospitalisation/death). There were 175,849 infections and 4113 hospitalisations/deaths of which 670 were deaths. In comparison with individuals who received dose 2 within the last 3 months, rVE in dose 3 recipients was 7% (95%CI 5-9%) against infection and 65% (95%CI 61-69%) against hospitalisation/death. Almost all dose 3 recipients had an mRNA vaccine; there was little difference in dose 3 rVE by primary course vaccine brand (ChAdOx1 versus BNT162b2). Over the 6-week study period, one hospitalisation/death was avoided for every 192 adults aged ≥70 years boosted with dose 3 in the infection-naiive cohort. The aHR for hospitalisation/death from Omicron was 0.12 (95%CI 0.07-0.23) for 2-dose recipients with a prior Delta infection compared with 2-dose recipients with no prior infection.

Interpretation: Receipt of a third COVID-19 vaccine dose significantly reduced hospitalisations and deaths during the first wave of SARS-CoV-2 Omicron infections in a primarily infection-naiive Australian population.

Funding: NSW Ministry of Health

Declaration of Interest: All authors have no conflicts of interest to declare

Ethical Approval: This study was conducted as part of NSW public health COVID-19 management and ethics approval was not required.

Keywords: SARS-CoV-2, Omicron, vaccine effectiveness, hybrid immunity

Suggested Citation

Liu, Bette and Gidding, Heather and Stepien, Sandrine and Cretikos, Michelle and Macartney, Kristine, Relative Effectiveness of COVID-19 Vaccination with 3 Compared to 2 Doses Against SARS-CoV-2 B.1.1.529 (Omicron) Among an Australian Population with Low Prior Rates of SARS-CoV-2 Infection. Available at SSRN: https://ssrn.com/abstract=4142075 or http://dx.doi.org/10.2139/ssrn.4142075

Bette Liu (Contact Author)

Children’s Hospital at Westmead - The National Centre for Immunisation Research and Surveillance (NCIRS)

Westmead
Australia

Heather Gidding

National Centre for Immunisation Research and Surveillance ( email )

Sandrine Stepien

National Centre for Immunisation Research and Surveillance ( email )

Australia

Michelle Cretikos

NSW Ministry of Health - Population and Public Health Division ( email )

Australia

Kristine Macartney

Children’s Hospital at Westmead - The National Centre for Immunisation Research and Surveillance (NCIRS) ( email )

Westmead
Australia