Back in January, there was an outside worship event at a Buddhist temple in China. 293 people attended, of which, 126 traveled to the temple in 2 buses.
• Bus ride: The ride was 50 minutes each way. The buses had an air-conditioned system, windows (that were kept shut), and there was no bathroom on board. Everyone remained seated and no one wore masks (this is before public awareness of COVID19). Everyone sat in their original seats on the ride back.
• The event: Once the buses arrived at the temple, worship took 150 minutes and the event included a luncheon (10 people at each round table). Bus riders were randomly mixed with others at the event. There was a slight breeze and everyone had close contact.
• Bus #1 (59 passengers+1 driver) had no index case.
• Bus #2 (67 passengers+1 driver) had an index case. The index case was asymptomatic. He sat in the middle of the bus (3rd seat in the 8th row).
• Bus #2 had an attack rate of 35%. In other words, 24 other passengers were infected during the 50-minute drive there or back. Bus #1 had an attack rate of 0% (even though they mingled with everyone at the event).
• Cases were scattered on Bus #2. There was no apparent pattern, other than distance from index case
• Among the worshipers that attended the event (but were not in one of these two buses), 7 tested positive (4% attack rate).
• There is airborne transmission with recycled air.
• In closed environments, COVID19 is highly transmissible.
• The index case, who was asymptomatic, achieved enough viral shedding by just breathing to cause a high secondary transmission rate
Translation: Even if you don’t feel symptoms, it’s important to wear a mask. You could be spreading the disease without even knowing it. And, be sure you’re in a space with air circulation (that is not recycled); open your window on the bus.
Data Source: Published 3 days ago in JAMA: https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/2770172