The Omicron variant is the current dominant circulating strain of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which is the causal agent of coronavirus disease 2019 (COVID-19). The World Health Organization has classified the Omicron variant as a variant of concern (VOC) as a result of its higher virulence and transmissibility as compared to the original SARS-CoV-2 strain.

Study: Effectiveness of COVID-19 Vaccination Against SARS-CoV-2 Omicron Variant in Two Outbreaks in Indoor Entertainment Settings in Australia. Image Credit: View Apart /


In Australia, the SARS-CoV-2 Omicron variant was first detected among passengers who arrived on an inbound international flight in November 2021. Subsequently, a rapid surge in COVID-19 cases was reported, particularly in the most populous state of New South Wales (NSW).

Prior to the introduction of the Omicron variant in Australia, low and stable COVID-19 cases of five for every 100,000 due to the Delta variant were reported. An impressive COVID-19 vaccination was reported in Australia.

The first large outbreaks of the Omicron variant in Australia occurred in NSW, in the city of Newcastle. During this time, entertainment venues required attendees to provide evidence of completed primary COVID-19 vaccination; however, compulsory wearing of face masks or restrictions on the number of individuals were not implemented. Later, healthcare officials found that two of the four outbreaks were related to entertainment venues.

One outbreak occurred on December 8, 2021, wherein an individual with the Omicron infection attended a nightclub that was then open to the public. The nightclub consisted of several interlinked rooms with few windows, with common areas of an average size of 1000 square meters with approximately one person per 2.5 square meters at peak occupancy.

A second outbreak occurred due to the ball organized on December 10, 2021, for recently graduated medical students and medical school faculty at the Exhibition Centre. The exhibition hall was 1,195 square meters and was exclusively ventilated using air-conditioning.

About the study

A new study posted to Preprints with The Lancet / SSRN´s First Look* estimates vaccine effectiveness against the SARS-CoV-2 Omicron variant in the aforementioned outbreaks in indoor entertainment settings in Australia. In this study, the scientists obtained the SARS-CoV-2 testing status of all individuals who attended each event and matched it to the Australian Immunization Register.

The authors classified these individuals based on having two or three valid doses of a COVID-19 vaccine. Furthermore, the researchers compared infection rates between the unvaccinated and vaccinated, in addition to estimating infection rate ratios and vaccine effectiveness.

Study findings

A high force of infection was observed among mostly young adults with two doses of a COVID-19 vaccine. In the nightclub environment, two doses of widely administered vaccines, such as the Pfizer-BioNTech, Moderna, and Astra Zeneca vaccines, had poor effectiveness against the SARS-CoV-2 Omicron infection. These vaccines were administered at least two months prior to the event date.

In the medical graduation ball, two doses of COVID-19 vaccines were moderately effective, with higher vaccine effectiveness for more recent vaccine receipts. The number of booster recipients was small and scientists did not observe any incremental effect of the booster over two doses of the vaccine.


The current study analyzed unique outbreaks and vaccine effectiveness in crowded indoor environments where social restrictions were absent. The results suggest that public health measures are needed in such settings, in addition to requiring that attendees have received at least two doses of available COVID-19 vaccines.

Other recent studies from the United Kingdom, Denmark, and Canada showed poor effectiveness of two doses of current COVID-19 vaccines against the Omicron variant. Some studies have also shown that the booster dose restores some protection against SARS-CoV-2 infection, which has prompted many countries, including Australia, to reduce the time interval for booster doses following primary vaccination.

The findings in this study are in line with previous research; however, researchers did not identify a significant incremental contribution of the booster dose in reducing the risk of SARS-CoV-2 infection, which was an unexpected result. Owing to the young demographic, the researchers were not able to estimate vaccine effectiveness against hospitalization or severe disease. About 2-3% of infected individuals were presented to a hospital emergency in the two weeks following the outbreaks.

Study limitations

One limitation of the current study is the small number of individuals with no vaccination, which made it difficult to base the estimates of absolute vaccine effectiveness and generalize the results to all social contexts. Furthermore, the scientists had access to limited data on potential confounders, including reasons for receipt of booster doses.

Not all SARS-CoV-2 isolates in the current study were confirmed as the Omicron VOC. About 70% of those in each setting had either whole-genome sequencing (WGS) or the indicative spike gene target failure (SGTF) reported.

*Important notice

Preprints with The Lancet / SSRN First Look publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:

  • Liu, B., Stepien, S., Pye, V., et al. (2022) Effectiveness of COVID-19 Vaccination Against SARS-CoV-2 Omicron Variant in Two Outbreaks in Indoor Entertainment Settings in Australia. Preprints with The Lancet.  doi:10.2139/ssrn.4026084


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