Effectiveness of COVID-19 Vaccination Against SARS-CoV-2 Omicron Variant in Two Outbreaks in Indoor Entertainment Settings in Australia

Background: We estimate vaccine effectiveness against the SARS-CoV-2 B.1.1.529 (Omicron) variant in two outbreaks in indoor entertainment settings: a nightclub and a graduation ball at an exhibition centre in Australia shortly after the emergence of Omicron.Methods: SARS-CoV-2 testing status of individuals registered as attending each event were obtained and matched to the Australian Immunisation Register. Individuals were classified as having 2 or 3 valid doses of a COVID-19 vaccine if dose receipt date was ≥14 days before the exposure date. Attack rates were compared between the unvaccinated and vaccinated, and rate ratios and vaccine effectiveness was estimated.Findings: Following exposure at each venue, 295/535 (55.1%) and 102/189 (54.0%) individuals tested positive for SARS-CoV-2 (mean: 5 days post-event). The majority had at least 2 valid vaccine doses: nightclub: 95.0% (508/535); ball: 97.9% (185/189). In the nightclub, vaccine effectiveness (VE) of at least 2 doses given approximately 2 months earlier was -11.8% (95%CI -74.5 to 28.3%). At the ball, VE of at least 2 doses given approximately 3 months earlier was 46.5% (95%CI 38.8 to 53.2%) but the unvaccinated reference group was small (n=3); VE was higher with shorter interval since second dose receipt, VE 87.5% (95%CI 64.0 to 95.7%), 60.0% (95%CI 38.0 to 74.2%) and 32% (95%CI 22.0 to 40.6%) at 1-<2, 2-<3 and 3+ months respectively; no differences in VE were found between 2 or 3 doses (p=0.94). In both settings there was no evidence of different effectiveness by vaccine brand (primary course of Pfizer, AstraZeneca, Moderna COVID-19 vaccines).Interpretation: The Omicron SARS-CoV-2 variant caused high attack rates in a two-dose vaccinated population in these high-risk indoor settings. Recent vaccination appears moderately effective against infection, but this varies depending on the setting, emphasising the importance of public health and social measures in addition to boosters to maximise protection.Funding: NSW HealthDeclaration of Interest: None to declare.Ethical Approval: This report was conducted as part of NSW Health public health surveillance and outbreak management and as such no formal ethical approval was required.