If the importance of aerosol transmission had been accepted early, we would have been told from the beginning that it was much safer outdoors, where these small particles disperse more easily, as long as you avoid close, prolonged contact with others. We would have tried to make sure indoor spaces were well ventilated, with air filtered as necessary. Instead of blanket rules on gatherings, we would have targeted conditions that can produce superspreading events: people in poorly ventilated indoor spaces, especially if engaged over time in activities that increase aerosol production, like shouting and singing. We would have started using masks more quickly, and we would have paid more attention to their fit, too. And we would have been less obsessed with cleaning surfaces.
Our mitigations would have been much more effective, sparing us a great deal of suffering and anxiety.
Since the pandemic is far from over, with countries like India facing devastating surges, we need to understand both why this took so long to come about and what it will mean…
Epidemiological studies and examples kept pouring in, too, all of them showing that Covid-19 was spreading primarily indoors and clusters were concentrated in poorly ventilated spaces. And when outdoor transmission did occur, it was often when people were in prolonged close contact, talking or yelling, as with construction workers on the same site.
The disease was also greatly overdispersed, sometimes being not very contagious and other times dramatically so. Large-scale studies showed that more than 70 percent of infected people did not transmit to any other person, while as few as 5 percent may be responsible for 80 percent of transmissions through superspreading events. Despite databases documenting thousands of indoor superspreader incidents, I’m not aware of a single confirmed outdoor-only case of superspreading.
Why did it take so long to understand all this?
The skepticism about airborne transmission is at odds with the acceptance of droplet transmission. Dr. Marr and Joseph Allen, the director of the Healthy Buildings program and an associate professor at Harvard’s T.H. Chan School of Public Health, told me that droplet transmission has never been directly demonstrated. Since Dr. Chapin, close-distance transmission has been seen as proof of droplets unless disproved through much effort, as was finally done for tuberculosis.
Another key problem is that, understandably, we find it harder to walk things back. It is easier to keep adding exceptions and justifications to a belief than to admit that a challenger has a better explanation.
In a contemporary example of this attitude, the initial public health report on the Mount Vernon choir case said that it may have been caused by people “sitting close to one another, sharing snacks and stacking chairs at the end of the practice,” even though almost 90 percent of the people there developed symptoms of Covid-19. Shelly Miller, an aerosol expert at the University of Colorado Boulder, was so struck by the incident that she initiated a study with a team of scientists, documenting that the space was less full than usual, allowing for increased distance, that nobody reported touching anyone else, that hand sanitizer was used and that only three people who had arrived early arranged the chairs. There was no spatial pattern to the transmission, implicating airflows, and there was nobody within nine feet in front of the first known case, who had mild symptoms.
Galileo is said to have murmured, “And yet it moves,” after he was forced to recant his theory that the earth moved around the sun. Scientists who studied bioaerosols could only say, “And yet it floats.”
So much of what we have done throughout the pandemic — the excessive hygiene theater and the failure to integrate ventilation and filters into our basic advice — has greatly hampered our response.