In a year of endless viral outbreaks, the details of the Princess Diamond tragedy seem like ancient history. On January 20, an infected passenger boarded the cruise ship; A month later, more than 700 of the 3,711 passengers and crew members tested positive, and many fell seriously ill. The invader moved as quickly and invisibly as the perpetrators on Agatha’s Christie’s East Express, leaving doctors and health officials with only piecemeal evidence to examine.
© Behrouz Mehri / Agence France-Presse – Getty Images
The Diamond Princess cruise ship, docked in Yokohama, Japan, in February. More than 700 of the 3,711 people on board tested positive for the coronavirus.
Since then, scientists have tried to pinpoint exactly how the coronavirus spread throughout the spacecraft. And for good reason: the Princess Diamond outbreak remains perhaps the most valuable case study available on coronavirus transmission – a data-rich experiment in a bottle, as well as a dark warning about what would come in large part. of the world.
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Now, researchers are beginning to use macroscopic tools (computer models that have revealed patterns in the global spread of the virus) to clarify the smaller-scale questions that currently dominate public discussions of security: how, exactly, does the virus move through? from a community, a building or a small group of people? What modes of transmission should we be most concerned about and how can we stop them?
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In a new report, a research team based at Harvard and the Illinois Institute of Technology have tried to decipher the ways the virus passes from person to person in Princess Diamond’s staterooms, corridors and common areas. She found that the virus spread more easily in microscopic droplets that were light enough to float in the air, for several minutes or much longer.
The new findings add to a growing debate among doctors, scientists, and health officials about the main routes of coronavirus transmission. Earlier this month, after pressure from more than 200 scientists, the World Health Organization recognized that the virus could remain in the air indoors, potentially causing new infections. Previously, he had emphasized only large droplets, such as a cough, and infected surfaces as the main drivers of transmission. Many clinicians and epidemiologists continue to argue that these routes are critical to the progression of the disease.
© Charly Triballeau / Agence France-Presse – Getty Images
A passenger on the Diamond Princess during a quarantine period in February.
The new findings, if confirmed, would have important implications for making interior spaces safer and choosing from a wide variety of personal protective equipment.
For example, ventilation systems that “change” or replace air in a room or building as often as possible, preferably using external air to do so, should make indoor spaces healthier. But good ventilation is not enough; The Diamond Princess was well ventilated and the air was not recirculating, the researchers noted. Therefore, it is likely that it will also be necessary to use good quality masks, standard surgical masks, or cloth masks with multiple layers rather than a single layer, even in well ventilated spaces where people keep their distance.
The computer model adds a new dimension of support to a body of accumulated evidence involving tiny droplets in the air in multiple outbreaks, including at a Chinese restaurant, a choir in Washington state, as well as a recent study of 13 evacuated passengers. of the Diamond Princess.
A researcher not involved in the new work, Julian Tang, an associate professor of respiratory science at the University of Leicester in the United Kingdom, said the document was “the first attempt, to the best of my knowledge, to formally compare the different routes of coronavirus transmission, especially short and long-range aerosols. “
He characterized the distances and type of particles involved with a simple analogy from everyday life: “If you can smell what I ate at lunch, you are getting my air, and you may also be getting virus particles.”
Another researcher, Linsey Marr, a professor of civil and environmental engineering at Virginia Tech who studies airborne virus transmission, had a more vivid description of the finding: the “garlic breath” effect.
“When you’re around someone, you smell that garlic breath,” said Dr. Marr. “Since you’re further away, you don’t smell it.”
The “garlic breath” effect would suggest that powerful ventilation in buildings, mainly using outside air or very well filtered, could reduce the transmission of the virus. The study found that small particles also had some ability to spread over longer distances, presumably beyond the range of breath odor.
Since the start of the pandemic, scientists have grappled with the spread mechanisms of the coronavirus. At first, surface transmission was widely emphasized; Now, several researchers prefer the larger droplets, which travel on more ballistic trajectories, like a stone in the air, and directly hit the mucous membranes.
There are other possibilities for candidates, too, said Dr. John Conly, an infectious disease physician and infection control expert at the University of Calgary in Canada, who has consulted with the World Health Organization.
“We are getting surprises all the time,” said Dr. Conly. “This article seems interesting to me, but I have a lot of time left to get into a line of credibility, in my opinion.”
Dr. George Rutherford, a professor of epidemiology at the University of California, San Francisco, was equally skeptical. Outside of hospital settings, he said, “Big drops in my mind account for the vast majority of cases. Aerosol transmission: If it really works with that, it creates a lot of dissonance. Are there situations where it could happen? Yes, maybe , but it is a small amount “.
Dr. Tang and other scientists strongly disagree. “If I talk to an infectious person for 15 or 20 minutes and inhale some of their air,” said Dr. Tang, “isn’t that a much easier way to explain transmission than to touch an infected surface and touch the eyes? When talking about an outbreak, like in a restaurant, the latter seems like a devious way to explain 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 at 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 long people spent in their cabins, on the terrace, or in the cafeteria, on average) and the amount of time the virus can live on surfaces. Each also factored into variable contributions of smaller floating droplets, broadly defined as 10 microns or smaller; and larger drops, which fall more quickly and infect surfaces or other people, by landing on your eyes, mouth or nose, for example.
Around 130 of those simulations replicated, to some extent, what actually happened on Princess Diamond as the outbreak progressed. By analyzing these more “realistic” scenarios, the research team calculated the most likely contributions for each transmission path. The researchers concluded that the smallest drops predominated and accounted for about 60 percent of new infections overall, both at close range, within a few meters of an infectious person, and at great distances.
“Many people have argued that airborne transmission is occurring, but no one had numbers for that,” said Dr. Azimi. “What is the contribution of these little drops? Is it 5 percent or 90 percent? In this document, we provide the first real estimates of what that number might be, at least for this cruise. “
The logic behind such a transmission is straightforward, experts said. When a person speaks, he emits a cloud of drops, the vast majority of which are small enough to remain suspended in the air for a few minutes or more. Through inhalation, that cloud of small droplets is more likely to reach a mucous membrane than the larger ones that are ballistically elevated.
Smaller drops are also more likely to penetrate deep into the respiratory system, into the lungs. It can take a much lower viral load (fewer viruses) to cause infection in the lungs than in the upper part, such as the throat. This, at least, is the case with other respiratory viruses, such as the flu.
Brent Stephens, a professor of engineering at the Illinois Institute of Technology in Chicago and co-author of the article, said the findings were important in determining, for example, what steps should be taken when college students return to campus.
The first, he said, should be “really enforce mask policies.” Another, he said, is recognizing that there is “great variability in mask quality,” and the material that really stops small sprays when someone breathes, talks, coughs or sneezes is crucial. Surgical masks are good, he said, but single-layer fabrics are often not.
As various transmission routes become clearer, they will provide specific guidelines on how to reopen schools, offices, restaurants, and other businesses.
“The value of this model is that it allows recommendations and guidance to be specific to each unique setting,” said another co-author, Joseph G. Allen, an expert on indoor air quality and an assistant professor at Harvard’s TH Chan School. Public health.
Dr. Allen said those settings ranged from restaurants to dental offices. In each case, he said, there are low-cost solutions that dramatically improve ventilation and filtration (most buildings are well below optimal levels) and, in turn, reduce the risks of infection in the air.
“For me, this is a complete moment,” said Dr. Allen. “We need better ventilation and better filtration, across the board, in all of our buildings.”
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