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Evidence of Long-Distance Droplet Transmission of SARS-CoV-2 by Direct Air Flow in a Restaurant in Korea

Our take —

This study highlights the risk of indoor dining, even when following COVID-19 safety protocols. While the index case (case A) was identified first, evidence from anemometer and CCTV readings show that most likely case B transmitted SARS-CoV-2 infection to case A and case C, each sitting 6.5m and 4.8m away from case B respectively. While it is not possible to confirm causality, using the Epidemic Investigation Support System which tracks COVID-19 cases throughout the country, this was the only exposure overlap between these cases where transmission likely occurred. Phylogenetic evidence supports transmission between these cases. This shows the importance of air flow management in indoor spaces and the risk for droplet transmission across longer distances than previously anticipated.

Study design

Case Series

Study population and setting

Following reports on June 17, 2020, that an index COVID-19 case in Jeonju, Korea, transmitted COVID-19 to two individuals 6.5m away (21 feet) in an enclosed space during 5 minutes of exposure in an air-conditioned restaurant, the Korea Disease Control and Prevention Agency began an epidemiological and environmental investigation. Patients were interviewed for sociodemographic and exposure data, medical facility use history and credit card use. Closed circuit television images of the exposure incident were reviewed, and cell phone location data were also used to assess exposure. Nasopharyngeal specimens of cases and close contacts were tested using real-time reverse transcription polymerase chain reaction (rRT-PCR) and genome sequencing was conducted. They also conducted an environmental analysis, which included recording dining table locations, timeline, type of air conditioning, distance between individuals, and the movement route of the index case and other people in the restaurant. Air speed of the air conditioning was measured with a portable anemometer, and 39 samples were taken from the environment to test for SARS-CoV-2 and analyzed with rRT-PCR.

Summary of Main Findings

The first identified case (case A) first developed symptoms on June 16, 2020, with a probable exposure as ascertained from their personal interview between June 2 and 15. Using the Epidemic Investigation Support Symptom (EISS), only one individual (case B) out of 538 domestic cases at the time was identified as having overlap with the index case based on the tracking map, at restaurant A on June 12. Using CCTV, the investigators concluded they spent at total of 5 minutes in proximity to one another. Among 13 close contacts of case A that had to quarantine for 14 days, one additional case was confirmed on June 20. Additionally, case B seems to have spread COVID-19 to case A and case C (two people total) at the restaurant. The investigators concluded this because case B typically lives in Daejeon City and only visited Jeonju on June 12th.The attack rate was 15.4% overall (2/13). Of the 39 environmental samples collected 11 days following the sentinel event, all tested negative by rRT-PCR. In the restaurant, the index case and case B were seated 6.5m from one another, and case B and case C were sitting 4.8m from one another. The anemometer measured a maximum airflow of 1.0m/sec between case A and B and 1.2 m/sec between case B and C. The study posits that the indoor air flow may have transmitted droplets from case B to case A and C based on the direction of the air conditioner flow.

Study Strengths

The study used a wide range of data sources, from the EISS, to the anemometer readings, to CCTV images, etc. By having a relatively low country-wide incidence and strong testing and contact tracing program, they were able to track each of these individuals and identify the potential contacts and modes of exposure with a high degree of certainty.

Limitations

The notable limitation was that the environmental samples collected were done so 11 days after the initial investigation began, which may have resulted in a negative finding at the time of collection. The study also did not engage in more sophisticated modeling of airflow dynamics, and cannot reflect the totality of factors (e.g., the impact of people opening doors, the impact of server movement throughout the restaurant), and therefore they cannot determine the causality of air flow dynamics in the restaurant. Finally, the study concluded that case B likely was the index case that transmitted the virus to case A and C, however that was determined by logical reasoning of their overlap in the restaurant compared to case B living in another city, but not because case B tested positive before cases A or C, or some other empirical evidence. Therefore, this conclusion relies heavily on assumptions that may not hold in the real world.

Value added

This case series shows the risk of indoor dining even with distancing and during very short periods of exposure.

This review was posted on: 26 February 2021