absence of evidence or evidence of absence?

It took two years for the WHO to admit covid is airborne. The reason is rooted in science history

A century ago, scientific theory labeled airborne transmission a form of superstition, and a belief held to this day
It took two years for the WHO to admit covid is airborne. The reason is rooted in science history
Photo: Noah Seelan (Getty Images)
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Covid, it’s now an established fact, is airborne.

Like other infectious diseases such as measles, chickenpox, or tuberculosis, it spreads through aerosols that can stay in the air for long periods of time, and travel long distances. The airborne quality of the virus is recognized by public health authorities including the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC).

Yet when the virus emerged, and for some time afterward, public health authorities thought instead that the virus might spread through large droplets, which unlike aerosols can only travel about two meters, and can fall on nearby surfaces, which in turn become potential vehicles of transmission. This was the theory that had everyone wiping down their groceries and wondering whether to disinfect their mail in early 2020.

Though aerosols and droplets may sound similar, their public health implications are very different. The assumption that covid was spread by droplets informed public health advice such as stressing the importance of social distancing, wearing any kind of mask including cloth ones, and disinfecting surfaces—as opposed to focusing on high-quality masks that can stop aerosol transmission (such as N95s) and ventilation.

But evidence that covid was airborne was abundant early on—what was lacking was the will to accept it. The WHO labeled the theory of airborne transmission as misinformation, and worked to dispel it, sharing on its social media channels: “#FACT: #COVID19 is NOT airborne.”

Yet as early as April 2020, a team of scientists presented evidence of aerosol transmission of covid to the WHO, gathered by studying the transmission dynamics of some superspreader events. “I said well, we will explain it to them, and then we’ll have a reasonable debate as scientists,” said Jose Jimenez, a professor of chemistry at the University of Colorado, Boulder, who was on the team led by renowned atmospheric physicist Lidia Morawska. But the WHO team, he said, was closed to the idea, and one of them was even rude and yelled at Morawska. It took almost two more years for the WHO to acknowledge she was right all along.

Jimenez, who had never worked on infectious disease before, was shocked. But his colleagues were not: The reaction they received tracked with a century-old history of public health opposition to the idea of aerosol transmission—which Jimenez set out to research for a paper published this week in the International Journal of Indoor Environment and Health.

From miasma to contact infection

For most of human history, the idea that illness would travel through the air was far from controversial. From Hippocrates’ miasma to Persian physician Ibn Sina (Avicenna), the first theories of how disease was transmitted involved vague concepts of bad fumes floating from unsanitary environments into people.

It wasn’t until the second half of the 17th century that the discovery of bacteria and microorganisms gave rise to an understanding that disease could also spread from person to person. What ensued, the paper recounts, was a long debate between so-called “contagionists” who thought that illnesses were spread from person to person, and “miasmatists” who believed in contagion through air. Subsequent discoveries, including germ theory, fed into one or the other camp until the turn of the century, and the work of Charles Chapin, an American epidemiologist.

Chapin’s research showed that germs could be transmitted through direct contact, but he found that the persisting belief in exclusively airborne transmission was an obstacle in containing diseases that spread through contact infection. “If the sick-room is filled with floating contagium, of what use it is to make much of an effort to guard against contact infection? […] It is impossible, as I know from experience, to teach people to avoid contact infection while they are firmly convinced that the air is the chief vehicle of infection,” he wrote in 1910 in his book, The Sources and Modes of Infection.

With more knowledge accumulated about the vehicles of diseases such as cholera (water) or malaria (mosquitoes), Chapin eventually came to the conclusion that airborne transmission was the most unlikely in most scenarios. As Jimenez and his colleagues put it in the paper, he turned the absence of evidence of airborne transmission into the evidence of its absence. In his writings, the paper reports, Chapin advocated for “discarding airborne transmission as a working hypothesis and devoting our chief attention to the prevention of contact infection.” He saw the paradigm shift as a historic step forward, and equated the idea of airborne transmission with little more than folklore. “It will be a great relief to most persons to be freed from the specter of infected air, a specter which has pursued the race from the time of Hippocrates,” he wrote.

This resulted in an overcorrection, led by the so-called reformers who adopted Chapin’s new paradigm. “They kind of showed that miasma was wrong and then they said, this isn’t going to be us, this is superstition, this is something we have to overcome,” said Jimenez.

Pesky little droplets

The rejection of aerosol transmission as a superstition carried a strong emotional charge that is still felt today. To explain disease transmission without direct contact, the preference went to the theory that pathogens would be carried through larger droplets—the same type initially blamed for spreading covid.

In fact, covid isn’t the first case where scientists had to fight their way to the admission of aerosol transmission. Droplets were initially deemed the cause of non-contact transmission of tuberculosis, measles, and chickenpox, and only after irrefutable proof would the scientific establishment concede the occurrence of aerosol transmission.

“Something that they told us at the WHO meeting is, ‘covid [was] not airborne like measles, if it was like measles we would notice.’ But actually measles and chickenpox, which are both extremely contagious airborne diseases, they were described as droplet form diseases until the 1980s,” said Jimenez.

What’s more, he said, while much effort is put into proving each instance of aerosol transmission, not enough attention is given to the fact that droplet transmission lacks substantial evidence. “Droplet transmission, which is what they told us and still tell us is the main mode of [covid] transmission, it has never been demonstrated directly—not just for covid, but for any disease in the history of medicine,” said Jimenez.

Aerosol scientists maintain that there is a physics misunderstanding in the very theory of droplet transmission, but the resistance among public health authorities is still strong. “Over the past two years, there has been substantial discussion regarding the modes of transmission of COVID-19, particularly the way SARS-CoV-2 is transmitted through the air. [...] This is a cross-cutting issue pertaining not only to SARS-CoV-2 but also to other respiratory pathogens capable of causing a public health emergency of international concern,” read a statement shared with Quartz by WHO’s spokesperson Margaret Ann Harris. “WHO is now leading and coordinating an international technical consultation process to debate and reach a consensus on this issue with global experts.”

Sickness in the air

Admitting to airborne transmission has implications that go beyond infectious medicine, or hard-held beliefs. If a disease is transmitted through direct contact, or by proximity, the responsibility to prevent it can be placed on the individual. Protective equipment, distancing, disinfecting: These are all measures that people can take to stop outbreaks. This way, getting sick becomes a personal failure—people must not have washed their hands, or missed some precaution.

But if the virus is airborne and one gets infected in a school or an office, where they can’t control the quality of the air, then the fault can’t be personal. Similarly, larger droplets can be stopped by any face covering, but if certain type of masks (such as N95 respirators) are required, then there is more of an institutional involvement in making sure they are affordable, available, and match certain quality criteria. “The institutions—the CDC, the government, the WHO—persist in the ambiguity because it is very convenient,” said Jimenez.

Correction: This article was amended to clarify only one person in the WHO team was rude and yelled at Lidia Morawska during the April 2020 meeting.