Plus, this debate has a long history: From the mid-19th century into the 20th century, infectious-disease specialists fought a long and hard-won battle against “miasma” theories of disease that posited that filth and noxious odors, instead of germs, were responsible for disease. In a seminal 1910 book, the public-health pioneer Charles Chapin distinguished “spray borne” diseases (WHO’s droplets that maximally travel only a few feet) from “dust borne” ones—spread by aerosols, or airborne transmission. He concluded that most pathogens were either “spray-borne” or spread through contact, and worried that an over-reliance on “air-borne” theories would needlessly scare the public or cause them to neglect hand-washing. More than a century later, there are still echoes of those concerns.
There are also different kinds of “airborne” transmission—the term can sound scarier than reality and can become the basis for unnecessary scaremongering. For example, some airborne diseases, such as measles, will definitely spread to almost every corner of a house and can be expected to infect about 90 percent of susceptible people in the household. In the virus-panic movie Outbreak, when Dustin Hoffman’s character exclaims, “It’s airborne!” about Motaba, the film’s fictional virus, he means that it will spread to every corner of the hospital through the vents. But not all airborne diseases are super-contagious (more on that in a bit), and, for the most part, the coronavirus does not behave like a super-infectious pathogen.
Read: Is the coronavirus airborne? Should we all wear masks?
In multiple studies, researchers have found that COVID-19’s secondary attack rate, the proportion of susceptible people that one sick person will infect in a circumscribed setting, such as a household or dormitory, can be as low as 10 to 20 percent. In fact, many experts I spoke with remarked that COVID-19 was less contagious than many other pathogens, except when it seemed to occasionally go wild in super-spreader events, infecting large numbers of people at once, across distances much greater than the droplet range of three to six feet. Those who argue that COVID-19 can spread through aerosol routes point to the prevalence and conditions of these super-spreader events as one of the most important pieces of evidence for airborne transmission.
Saskia Popescu, an infectious-disease epidemiologist, emphasized to me that we should not call these “super-spreaders,” referring only to the people, but “super-spreader events,” because they seem to occur in very particular settings—an important clue. People don’t emit an equal amount of aerosols during every activity: Singing emits more than talking, which emits more than breathing. And some people could be super-emitters of aerosols. But that’s not all. The super-spreader–event triad seems to rely on three V’s: venue, ventilation, and vocalization. Most super-spreader events occur at an indoor venue, especially a poorly ventilated one (meaning air is not being exchanged, diluted, or filtered), where lots of people are talking, chanting, or singing. Some examples of where super-spreader events have taken place are restaurants, bars, clubs, choir practices, weddings, funerals, cruise ships, nursing homes, prisons, and meatpacking plants.