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Aboard the Princess of Diamonds, Aerosol Transmission Case Study



“We’re having surprises all the way,” Dr. Conly said. “In this paper I find it interesting, but I still have to go so I can get into a line of credibility.”

Dr. George Rutherford, a professor of epidemiology at the University of California, San Francisco, was similarly skeptical. He said that, outside of hospital settings, “big drops in my mind account for the vast majority of cases. Aerosol transmission – if you really work with that, it creates a lot of dissonance. Are there situations where there may be? Yes maybe, but it’s a small amount. “

Dr. Tang and other scientists disagree. “If I’m talking to an infected person for 1

5 or 20 minutes and to get some air out of them,” Dr. Tang said, “isn’t that a simpler way to explain transmission than touching an infected face and touching your eyes? When you’re talking on an outbreak, as in a restaurant, the latter seems to be a torturous way of explaining the transmission. “

In the new analysis, a team led by Parham Azimi, an indoor air researcher at Harvard’s TH Chan School of Public Health, studied the outbreak on Princess Diamond, where physical spaces and infections were well documented. He ran more than 20,000 simulations of how the virus could have spread throughout the ship. Each simulation made a variety of assumptions, about factors such as patterns of social interaction – how much time people spend in their booths, on the deck or in the cafeteria, on average – and the amount of time the virus can live on surfaces. Each also considered in various contributions of small, floating droplets, broadly defined as 10 microns or smaller; and larger drops, which fall faster and infect other faces or people, by falling on their eyes, mouth or nose, they say.

About 130 of those simulations reproduced, to some extent, what actually happened to the Princess of Diamonds as she burst. By analyzing these most “realistic” scenarios, the research team calculated the most likely contributions of each transmission route. The researchers concluded that small drops predominated, accounting for about 60 percent of new infections above all, both in nearby locations, in a few yards of an infected person, and at greater distances.

“A lot of people have argued that airborne transmission is happening, but no one had numbers for that,” Azimi said. “What is the contribution from these small drops – is it 5 percent, or 90 percent? In this paper, we provide the first real estimates for what that number might be, at least in the case of this cruise ship. “

The logic behind this transmission is simple, experts said. When a person speaks, he or she emerges from a cloud of drops, the vast majority of which are small enough to remain suspended in the air for a few minutes or longer. By inhalation, those small droplet clouds are more likely to reach a mucous membrane than larger ones that ballistically rise.


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