Disease & Genomics

Why Indoor Transmission of COVID-19 is Remarkably High

COVID-19 and the workplace

In the wake of the COVID-19 pandemic, public health experts and governments implemented several measures, including social distancing, restriction on gatherings, and restriction on indoor activities to curb the spread of the virus. As the outbreak continued to spread, researchers found that clusters and surges of the infection were traced to indoor gatherings. They reported that adults with positive COVID-19 test results were twice as likely to have reported dining at a restaurant than those with negative test results. However, there’s something more intriguing about these findings: the risk of transmission is still high beyond the current distance guidelines and with exposures as short as five minutes.

A striking study released from South Korea perfectly describes these nuances of COVID-19 transmission in indoor settings.

The study authors examined how the virus was transmitted between three diners, who were at distances much farther than the 6 feet guideline. The study authors identified the three persons involved as cases A, B, and C respectively.

The index case, case A, was identified in Jeonju. Contact tracing data showed that the person had no history of travel outside the city. Then, using contact-tracing and location data, they tracked his itinerary to case B and the location where the infection overlap happened - a restaurant at the city center.

The study authors visited the restaurant to investigate the occurrence and also test everyone who worked there and who was present there on the day of the overlap. They found one more infected person, case C.

The investigators watched CCTV footage in the restaurant to observe table locations, timelines, and the movement route of case A and everyone else in the restaurant. They also measured the distance between tables, assessed the sitting positions of the diners, and measured the speed and direction of air from the ceiling type air conditioners using a portable anemometer. The investigators collected a total of 39 environmental samples of inlets and outlets of air conditions, table seat of case A, and nearby tables and chairs while considering the flow of air in the room.

After confirming that there were no other interactions in the room, the researchers conducted genome sequencing of the three patients to confirm that the viral transmission did not happen anywhere else but within the restaurant. The investigation confirmed that case A infected both case B and case A within the restaurant from a distance of 6.5 meters (21 feet) and 4.8 meters (15 feet) away. Case B and Case A overlapped for about five minutes at that distance, well beyond the 6 feet/2 meter CDC recommendation and the current 3 feet/ 2 meter WHO recommendation.

The study also confirmed one more interesting find: that case A infected others the day before his symptoms began - suggesting that asymptomatic patients may be the main drivers of the outbreak.

These findings substantiate the results of an earlier enlightening study about the risk of indoor dining. In this study, also, one person (case A1) infected 10 other people in a restaurant in Guangzhou. In this study, too, CCTV footages confirmed that the airflow speed and direction were the major factors that enabled the spread. Here, too, people were infected at distances farther from the recommended guideline of 6 feet.

Therefore, these studies confirm something interesting: airborne transmission of COVID-19 in indoor settings defies current distance guidelines and the definition of close contact (contact with an infected person within less than 6 feet away for longer than 15 minutes).

Further, in a sharply contrasting twist, those who sat close to the infected persons did not get infected. In this case, the people who sat next to the infected person backed the airflow. The study authors noted that everyone who was not directly facing the airflow was spared regardless of the distance they were from, and how long they sat next to the infected case. In summary, the closer you are to an infected person, the more likely you are to get hit by the infection; however, your risk is largely determined by your position and the airflow within the room.

These studies, again, indicate that the risk of transmission is not the same in all indoor settings. Examining the Skagit County choir case, researchers said some indoor activities could increase the risk of spread much more than others. The initial report from the case revealed that 52 people out of 61 in one choir practice were infected by a single person. Even after implementing some precautions, including placing hand-sanitizers around and restricting physical contact, a large outbreak occurred. The scientists tied the outbreak to one single activity - singing. This find corroborates previous studies that explain how singing releases much more aerosols than talking and breathing.

These findings are enlightening and they give us a clue as to how the virus spreads and ways we could effectively curb it. It is safe to say that limiting indoor gatherings lowers your risk of getting the infection, but indoor gatherings do not carry equal risks — and it does not matter how many people are within the room. With adequate social distancing, hand hygiene, and a limit on the number of people in a gathering, one person can still spread the infection to nearly all within the room.

This reality suggests that face masks and proper ventilation are crucial tools for limiting COVID-19 transmission. Masks both dampen the emission of infected droplets and aerosols from an infected person and protects one from inhaling them. Well-positioned Ventilation systems with high-efficiency filters help to clean the air, eliminating particulate matter and recirculating fresh, outside air into the room.

These data are useful for business owners and HR leaders as they prepare their office buildings for a return to on-site work. It’s not just about social distancing and limiting the number of workers in the offices; identifying and isolating an infected worker, ensuring proper ventilation in your offices, and implementing a mask mandate are your critical tools to keeping the indoor transmission of COVID-19 at bay. 

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