From January to February 2020, 10 people from 3 families were infected by Covid-19 after eating in an air-conditioned restaurant. Here is how it happened.
The restaurant is 145 m². The tables were about 1 meter apart. There is an air-conditioned, as shown in the figure below, next to table C. An exhaust fan is near Table B.
On January 24, there were 91 people in the restaurant at that time. Out of the 91, 10 were later diagnosed with Covid-19. The 10 cases were from family A, B, and C. (Big circles in figure below)
There was an overlap period of 53 minutes for families A and B and 73 minutes for families A and C during that time.
Family A traveled from Wuhan and had lunch at a restaurant. At that time, A1 was asymptomatic. Later that day, A1 developed fever and cough and went to the hospital and was diagnosed with Covid-19. A1 is the source or index case for all the 10 future Covid-19 patients from that restaurant.
The 10 sick people belong to families A, B, and C. The letters with numbers in small circles are the Covid-19 cases. Family A1-A5, Family B1-B3, and Family C1-C2. The dates next to the red circles are the dates they became symptomatic. (see figure below).
The authors suspect that B1 and C1 may have been initially infected and then spread the virus to their other family members.
The 6 smear samples from the air conditioner (3 from the air outlet and 3 from the air inlet) were all negative for the Covid-19 virus. The authors did not say when the smears from the air conditioners were obtained.
In this study, the authors conclude that the Covid-19 viruses that infected the other people were from A1 and was transmitted by droplets. The droplets were then propagated by the strong airflow from the air-conditioner and spread the droplets from Table A to Table B and then back to Table C.
My humble opinion
I think the spread of Covid-19 in this restaurant is more of an aerosol than droplets. Air-conditioners cool the air by removing humidity. This results in drier air.
When people eat, the increased fluid intake tends to make any mucus in the throat and nasal passages more watery. This makes any nasal or throat secretions and saliva come out readily by talking.
It is most likely that Patient A1 was happily chatting with the other family members during lunch. A previous article showed that talking in a normal voice can expel millions of viruses thru droplets and aerosol.
Some droplets surrounding the germs will dry up in the air and make the virus travel further. The Covid-19 virus is stable in the air for 3 hours.
The concentration of virus particles in the air determines infectivity. The smaller the environment or, the nearer to the source of infection, the higher the viral density.
Patient A1 is reported as asymptomatic during that lunch and had symptoms hours later. Research has shown that the multiplication of Covid-19 viruses happens during the asymptomatic period. During that lunchtime, A1 was dishing out the germ, and the air conditioner did the rest of the spreading.
That is how 9 other people got infected.
Take Away Message
Be aware of where you sit. Do not be downwind from anyone that may be an asymptomatic carrier. The airflow may come from a window, an electric fan, or even outdoors. Take-out may be an alternative.
A distance of 6 feet may not be enough to prevent a wind carried droplet or aerosol transmission.
Wear a mask.
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Don’t Get Sick!
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Reference:
Lu J, Gu J, Li K, Xu C, Su W, Lai Z, et al. COVID-19 outbreak associated with air conditioning in restaurant, Guangzhou, China, 2020. Emerg Infect Dis. 2020 Jul [date cited]. https://doi.org/10.3201/eid2607.200764. DOI: 10.3201/eid2607.200764
Image Credit: from COVID-19 outbreak associated with air conditioning in restaurant, Guangzhou, China, 2020. Emerg Infect Dis. 2020 Jul
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