High attack rate of SARS-CoV-2 B.1.1.529 among two-dose vaccinated populations in two indoor entertainment setting outbreaks.

We estimated attack rates of SARS-CoV-2 B.1.1.529 (Omicron) infection among people attending a nightclub and a graduation ball where >95% had at least 2 vaccine doses. Attack rates were 295/535(55.1%) and 102/189(54.0%) respectively (mean: 5 days post-event). At the ball, attack rates increased with time since vaccination: 12.5% among those vaccinated 1-2 months previously; 68.0% among those vaccinated 3 + months previously; such differences were not found at the nightclub. Recent vaccination prevents Omicron infection, but is time and setting-dependent, emphasising the importance of non-pharmaceutical public health measures in addition to vaccine booster doses to maximise protection in high-risk contexts.