Outbreak Investigation of Airborne Transmission of Omicron (B.1.1.529) - Sars-Cov-2 Variant of Concern in a Restaurant : Implication for Enhancement of Indoor Ventilation

Airborne transmission of SARS-CoV-2 has been increasingly recognized in the outbreak of COVID-19, especially during the emergence of Omicron variant. We conducted an investigation of a restaurant outbreak due to Omicron variant. Besides the epidemiological and phylogenetic analysis, the secondary attack rates of customers of restaurant-related COVID-19 before (outbreak R1) and after enhancement of indoor ventilation (outbreak R2) were compared. On 27th December 2021, an index case attended a restaurant R2 for 98 minutes. Except for 1 sitting in the same table, six other secondary cases sat in 3 corners at 3 different zones, where designated staff served customers at different zone. The median exposure time was 34 minutes (range:19-98 minutes). All 7 secondary cases were phylogenetically related to the index. Smoke test demonstrated the direction of airflow which explained the distribution of secondary cases. Compared with an earlier COVID-19 outbreak in another restaurant R1 (19th February 2021), just preceding the mandatory requirement of enhancement of indoor ventilation, the secondary attack rates among customers in R2 was significantly lower than that in R1 (3.4%,7/207 vs 28.9%,22/76,p<0.001). Enhancement of indoor ventilation with installation of air purifier and upper-room ultraviolet-C germicidal irradiation could minimize the risk of SARS-CoV-2 transmission in the restaurant