How Richard Riley’s findings about tuberculosis transmission inform our COVID-19 response today.

Seven months after the first human-to-human transmission of SARS-CoV-2 was confirmed in China, the method of transmission remains unsettled. Is the virus airborne in tiny infective particles? Or do infected people expel the virus in large droplets that fall on surfaces or can be inhaled by people nearby?

Current control measures—washing hands, maintaining a 6-foot distance from other people, disinfecting surfaces—are based on the latter theory. But in July, 239 scientists—including Johns Hopkins engineer Peter DeCarlo—published an open letter urging the World Health Organization and medical authorities to “recognize the potential for airborne spread of COVID-19.” The signatories claimed to have “demonstrated beyond any reasonable doubt that viruses are released during exhalation, talking, and coughing in microdroplets small enough to remain aloft in air and pose a risk of exposure.”

All the studies of airborne coronavirus transmission to date have used observational data and epidemiological modeling because conducting scientific experiments to detect tiny infective particles in large volumes of air is extremely difficult. In fact, there has been only one replicable, controlled experiment that definitively proved airborne spread of infectious respiratory disease.