Airborne Transmission of COVID-19: Epidemiologic Evidence from An Outbreak Investigation

SARS-CoV-2, the pathogen causing COVID-19, first emerged in humans in late 2019 in Wuhan, China. Much remains unknown about COVID-19 transmission dynamics hampering the ability to implement effective preventive interventions. Understanding the mechanisms of COVID-19 transmission is critical to the prevention and containment of the disease. Several case studies have demonstrated transmission routes through close contact and respiratory droplets produced when an infected person coughs or sneezes. Evidence on whether SARS-CoV-2 can be transmitted as an aerosol (i.e., airborne) is highly sought after, and has profound public health implications during the global pandemic. To inform the knowledge gap and investigate potential transmission routes of COVID-19, we present the result from an outbreak investigation of COVID-19 among lay Buddhists worshiping in a temple. The attack rate on an enclosed bus containing an individual with COVID-19 was 34.3% while no one on a second similar bus was infected. The natural experiment provided convincing evidence that the disease transmission largely occurred during the bus rides. Through further epidemiologic investigation, we determined that the possibility of transmission by direct contact on the exposed bus was low. The absence of a significantly increased risk in the part of the bus closer to the index case suggested that the risk was fairly evenly distributed throughout, and thus, airborne spread of the virus may, at least partially, explain the extraordinarily high attack rate observed. This finding has important public health significance and future efforts at prevention and control should consider the potential for airborne spread of COVID-19, particularly in enclosed spaces with re-circulating air conditioning systems.