One common COVID-19 emergency shutdown order is the closure of indoor dining in restaurants in some U.S. states. However, some other states allow indoor dining, so long as 6 feet of distance can be maintained between tables. So, naturally we may wonder, which state has it right? Well one recently published scientific study may provide some insight.
First, let’s review what we know about COVID-19 transmission [1]. The SARS-CoV-2 virus is mainly transmitted through droplets that are emitted when an infected person coughs, sneezes, talks or breathes. These droplets typically settle within 6 ft (2m) of their origin due to gravity[2], hence the “social distancing” guideline of 6 ft.
In the study “Evidence of Long-Distance Droplet Transmission of SARS-CoV-2 by Direct Air Flow in a restaurant in Korea,” the authors examined a COVID-19 outbreak in South Korea that was traced back to a restaurant with air conditioning [2]. The outbreak consisted of four individuals, who we will call Case A, Case B, Case C and Case D in the order they were determined. Case A tested positive on June 17, 2020, in Jeonju, Korea, which was the first positive case in the region in two weeks. As Case A had no history of travel outside the city, the epidemiological investigation team was able to use contact tracing to determine that Case A was infected at a restaurant on June 12, by Case B. Case B had traveled to the same restaurant from another city (Dajeon), located about an hour away. Case B infected two total individuals at the restaurant: Case A and Case C, which amounted to an attack rate of 15.4% (2/13 people in the restaurant were infected). Later, it was determined that Case B’s companion also tested positive (Case D) after having previously been exposed to another epidemic patient in Dajeon city on June 11, but it is not believed they spread COVID-19 to others at the restaurant.
What makes this infection rate so surprising, however, is that Case B did not closely interact with either Case A or Case C, according to the CCTV footage at the restaurant. Case B sat about 21 feet (6.5 meters) away from Case A. Case A had finished their meal before Case B (and Case D) arrived at the restaurant, and left within 5 minutes of Case B arriving. During this 5-minute overlap, Case A and Case B were conversing with their companions at their table without masks. A few minutes after Case A left, Case C and their companions arrived, and sat 15.7 ft (4.8 m) away from Case B. Case B left 21 minutes after Case C arrived. The maximum airflow caused by the air conditioning system from Case B to each infectee ranged from 3.2-3.7 ft/s (1-1.2 m/s). Genome sequencing for Case A, Case B, and Case C demonstrated that all three cases had the same genomic type of SARS-CoV-2.
Schematic of restaurant [2]: In this schematic, the solid arrow lines represent the airflow within the restaurant. Some lines are curved, as the airflow is reflected off the wall or barrier, and moves downward to the floor.
There are a few key conclusions we can draw from this:
1. Indoor air flow can cause air droplets carrying COVID-19 particles farther than 6 ft (2 m), causing infection in a period of exposure as short as 5 minutes
2. The attack rate among exposures at the restaurant was 15.4%, which was higher than the secondary attack rate among total close contacts (0.3-1.0%) and only household contacts (3.7-14.3%) in Korea
3. Contact tracing is a useful tool for identifying the infection source and quickly determining potential cases, thus reducing the overall burden for testing, isolation and treatment
Summary of COVID-19 transmission in the restaurant [2]: This diagram summarizes the time and distance from which Case B infected Case A and Case C
One thing we cannot conclude from this study, however, is if COVID-19 is spread through airborne transmission. As mentioned before, although droplets typically settle within 6 ft, however this is in the absence of airflow. Given airflow in the form of air conditioning or wind, droplets can travel much further.2 Case A and Case C were both in the direct air path from Case B and became infected. However, visitors that were closer to Case B for a longer period were not infected. Additionally, the companions of Case A and Case C were not infected, because they faced away from the infector’s face. These observations all support droplet transmission rather than airborne transmission. Airborne transmission, on the other hand, would involve small droplets, or “aerosols” (<5µm) remaining suspended in the air over a long period of time. While some studies show evidence that airborne transmission of COVID-19 is possible, especially in indoor and crowded settings, this topic requires further investigation [1, 2].
There are some ways that long-distance droplet transmission can be mitigated. One way is to determine the location of fans or air conditioners and arrange seating to avoid direct airflow between two tables. Another way is to ventilate the indoor space frequently. Additionally, wind partitions could be installed to prevent airflow between tables. Masks can also reduce the spread of COVID-19, and should only be removed for eating.
So what does this mean for shutdowns—which states have it right? Given that COVID-19 can be transmitted over distances greater than 6ft (2m) in indoor settings, mitigation measures reduce but not eliminate the risk, and the current number of COVID-19 cases in the U.S., it is probably best to stick to take-out!
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Works Cited:
[1] World Health Organization. Coronavirus disease (COVID-19): How is it transmitted? Web. 2020 July. https://www.who.int/news-room/q-a-detail/coronavirus-disease-covid-19-how-is-it-transmitted
[2] Kwon KS, Park JI, Park YJ, Jung DM, Ryu KW, Lee JH. Evidence of Long-Distance Droplet Transmission of SARS-CoV-2 by Direct Air Flow in a Restaurant in Korea. J Korean Med Sci. 2020 Nov;35(46):e415. https://doi.org/10.3346/jkms.2020.35.e415
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