Scientists warn normal speech by asymptomatic individuals can infect others with COVID-19

Researchers at UC Davis published an article in the Journal of Aerosol Science & Technology last Thu (26 Mar) pointing out that normal speech by individuals who are asymptomatic but infected with coronavirus may produce enough aerosolized particles to transmit the infection (‘The Coronavirus Pandemic and Aerosols: Does COVID-19 Transmit via Expiratory Particles?‘).
UC Davis also followed up by publishing an article in its university news on Thursday, quoting the research paper (‘How Important Is Speech in Transmitting Coronavirus?‘, 2 Apr).
Aerosols are particles small enough to travel through the air. Ordinary speech creates significant quantities of aerosols from respiratory particles, said Prof William Ristenpart, professor of chemical engineering at UC Davis.
These respiratory particles are about one micron in diameter. That’s too small to see with the naked eye, but large enough to carry viruses such as influenza or SARS-CoV-2, the COVID-19 virus.
Last year, Prof Ristenpart and colleagues published a paper showing that the louder one speaks, the more particles are emitted and that some individuals are “superemitters” who give off up to 10 times as many particles as others. The reasons for this are not yet clear. The researchers have also investigated which speech sounds are associated with the most particles.
In airborne transmission, the researchers noted 2 distinct modes, which require no physical contact between infected and susceptible individuals. During a sneeze or a cough, “droplet sprays” of virus-laden respiratory tract fluid, typically greater than 5 micron in diameter, impact directly on a susceptible individual. Alternatively, a susceptible person can inhale microscopic aerosol particles consisting of the residual solid components of evaporated respiratory droplets, which are tiny enough (smaller than 5 micron) to remain airborne for hours.
People with COVID-19 but without symptoms can infect others
So far, much airborne disease research prior to the current pandemic has focused on ‘violent’ expiratory events like sneezing and coughing. But there is strong evidence now that many infected individuals who transmit COVID-19 are either minimally symptomatic or not symptomatic at all.
Indeed, in Singapore, for example, a recent study published by the United States’ Centres for Disease Control and Prevention and co-authored by Associate Professor Vernon Lee, director of communicable diseases at the Ministry of Health (MOH), revealed that at least 10 people in Singapore were infected by the coronavirus from those without symptoms (‘At least 10 people in SG got COVID-19 from those without symptoms at point of infection‘, 3 Apr).
The UC Davis researchers also noted that epidemiologists led by Prof Shaman of Department of Environmental Health Sciences from Columbia University had calculated that about 86% of infections in Wuhan prior to the implementation of travel restrictions, were “undocumented” individuals, those with “mild, limited, or no symptoms” who accordingly were never tested. Notably, their modeling indicated that 79% of the actual documented cases were infected by undocumented individuals. Furthermore, inspection of the average delay time between infection and initial manifestation of symptoms led them to conclude that “…pre-symptomatic shedding [of virus] may be typical among documented cases.”
In other words, it appears that large numbers of patients who became ill enough to require hospital treatment could have themselves been infected by others who did not appear sick.
Both ordinary breathing and speech can emit large quantities of aerosol particles
The question remains, if pre- or asymptomatic infected individuals do not sneeze or cough, how do they generate aerosols? In fact long ago, it was established that ordinary breathing and speech both emit large quantities of aerosol particles, the researchers noted. These expiratory particles are typically about 1 micron in diameter, and thus invisible to the naked eye; most people unfamiliar with aerosols are completely unaware that they exist. The particles are sufficiently large, however, to carry viruses such as SARS-CoV-2, and they are also in the correct size range to be readily inhaled deep into the respiratory tract of a susceptible individual.
Recent work on influenza has also established that viable virus can indeed be emitted from an infected individual by breathing or speaking, without coughing or sneezing, the reseachers highlighted.
With regard to the “superemitters”, it was established that they can emit an order of magnitude more aerosol particles than average, about 10 particles/second. A ten-minute conversation with an infected, asymptomatic “superemitter” talking in a normal volume thus would yield an invisible “cloud” of approximately 6,000 aerosol particles that could potentially be inhaled by the susceptible conversational partner or others in close proximity.
The researchers think that given the large numbers of expiratory particles known to be emitted during breathing and speech, and given the clearly high transmissibility of COVID-19, a plausible and important hypothesis is that a face-to-face conversation with an asymptomatic infected individual, even if both individuals take care not to touch, might be adequate to transmit COVID-19.