Covid Summaries

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11:13a, 9/18/20

This is the section that some will consider controversial

Aerosol transmission has gradually gained acceptance

Despite these observations, some public health experts were initially reluctant to say that the virus is airborne, partly because they didn't want to alarm the public. There are also debates between epidemiologists, virologists, and aerosol engineers about what the word airborne really means whether the size of the particles or their behavior (how quickly they fall to the ground, whether they can be carried on a gust of air) matters more, and what questions must be answered before a disease can be defined as such.

Part of the resistance to calling Covid-19 airborne is also rooted in history. For centuries, doctors and scientists didn't know how diseases spread. One theory was that infections traveled in invisible clouds called miasmas or "bad air." It wasn't until the 1860s that Louis Pasteur's germ theory of disease began to take hold, cemented in the 1890s with the discovery of viruses. As a result, scientists waged a campaign during the early 20th century to discredit the idea of miasmas and airborne spread with the goal of getting the public to take germs and personal hygiene seriously.

"That became the paradigm of epidemiology and infectious diseases from 1910 until now," says Jose Luis Jimenez, PhD, a professor of chemistry at the University of Colorado, Boulder who specializes in aerosols. "For medicine, during all this time, a disease going through the air is extremely difficult. It's an outlandish proposition."

As a result of this legacy, public health experts initially believed that SARS-CoV-2 couldn't be spread through the air because the presumption was that virtually no diseases were. There have been a few exceptions made over the years, but those were for viruses that are so contagious they couldn't conceivably be spread any other way namely, measles and chickenpox.

"For diseases like measles and chickenpox, because they are extremely transmissible, the evidence became too obvious," Jimenez says. "They're so transmissible through the air that it just became undeniable, and they were accepted as transmitted through aerosols."

As surprising as it may sound, by comparison, the novel coronavirus is not very contagious. Each person who gets infected with SARS-CoV-2 will, on average, spread it to two or three other people. A person with measles will infect 15 others. Jimenez says the WHO initially cited the coronavirus's relatively low infectious rate as a reason why it couldn't be spread through the air. "[They] are confusing an artifact of history with a law of nature," Jimenez says. "They are thinking it is a law of nature that if a disease goes through the air, it has to be extremely contagious."

It wasn't until a public outcry from over 200 scientists that the WHO finally conceded in July that aerosol transmission was possible.

So if the novel coronavirus is airborne, why isn't it as contagious as measles? One reason could be that measles is a heartier virus (remember that SARS-CoV-2 is relatively fragile) and can survive longer in those tiny aerosols. Another potential difference is the infectious dose the amount of virus required to start an infection. Scientists still don't know exactly how much of the novel coronavirus is needed to make someone sick, but it's likely higher than conventional airborne viruses.

"What's the infectious dose via the respiratory route is really probably the last piece of this that isn't completely answered yet," says Joshua Santarpia, PhD, an associate professor in the department of pathology and microbiology at the University of Nebraska. "I think people have this preconceived notion that if it's airborne it's like the measles or like smallpox where it only takes one viral particle to infect you, and this is almost certainly not the case with this coronavirus. Most coronaviruses are probably in the hundreds."

Another question that needed to be answered before many public health experts could accept that SARS-CoV-2 was airborne was whether it could even survive in those smaller aerosol particles. Some viruses can't because they dry up too quickly without a larger liquid droplet to support them. However, many scientists feel this issue has been put to rest with two recent papers (which have yet to be peer-reviewed) that provide what some have called the "smoking gun" for aerosol transmission: live, replicating virus collected from the air of Covid-19 patient hospital rooms.

"Confidently, what you can say is that things that we consider aerosols, not droplets, have both [viral] RNA and [live] virus in them that is capable of replication in cell culture," says Santarpia, who led one of the studies. "I think that between the two of [our studies], you can say that aerosols are infectious meaning that probably we're looking at something that's airborne."

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