The primary mode of transmission of the novel coronavirus is believed to be through direct contact with the respiratory fluids of an infected person. A new study suggests that normal speech by individuals who have COVID-19 but are asymptomatic may still produce enough aerosolized particles to transmit the infection.

The study, however, does not tell us how frequent or important this mode of transmission is to the spread of COVID-19, nor has the hypothesis been verified by experiments.

Although there are many unknowns regarding SARS-CoV-2, the novel coronavirus that causes COVID-19 and which first appeared in Wuhan, China in late 2019, two things seem eminently clear: the virus is both extremely contagious and deadly.

The coronavirus is insidious in its modes of transmission

Dissecting the modes of contagion for the COVID-19 is, however, challenging. When an infected person reports to the hospital, doctors can perform contact tracing in order to track individuals who came into close contact with the patent under investigation. However, such an investigation cannot inform you how the virus itself transferred to the patient.

SARS-CoV-2 can infect humans through direct or indirect contact. Direct contact refers to person-to-person transmission, which requires a susceptible individual to physically touch their mouth, nose, or eyes after, for instance, transferring the virus between an infected and susceptible host through a handshake. Indirect contact modes require a susceptible host to come into contact with a “fomite” — an object that was previously contaminated by an infected host.

Not much is known about the role played by aerosol transmission in the spread of COVID-19, which requires absolutely no physical contact between infected and susceptible individuals.

When an infected person sneezes or coughs, droplets of virus-laden respiratory tract fluid are expelled, which can infect a susceptible host. These particles are typically 5 micrometers in diameter. Alternatively, a susceptible host may inhale aerosol particles consisting of the residual solid components of evaporated respiratory droplets, which are smaller than 5 micrometers.

Some viruses can infect people through aerosol transmission. Aerosols are particles small enough to travel through the air. In the case of SARS-CoV-2, it’s droplets that are the favored medium of propagation.

Studies in China suggest that about 80% of documented cases were infected by undocumented individuals, meaning those with “mild, limited, or no symptoms”. There is also evidence that suggests pre-symptomatic shedding of viral material may be typical among documented cases.

These findings suggest that a large number of patients become ill enough to require hospital treatment may have been infected by others who do not appear sick.

It’s not clear whether people can get infected when an infected person expels aerosols through speech — but it is a plausible hypothesis

A new study performed by researchers at the University of California, Davis, examined the role that asymptomatic cases might have on the spread of the disease during normal speech.

It is an established fact that normal speech creates aerosols from respiratory particles. Such particles are around one micrometer in diameter, making them too small to see with the naked eye, but large enough to piggyback viruses such as influenza or SARS-CoV-2.

Last year, William Ristenpart, professor of chemical engineering at UC Davis, published a paper showing that the louder one speaks, the more aerosols are emitted. Some individuals produce more aerosols than others, up to 10 times as many particles as others in the case of “super-emitters”. The team published another study that investigated the effect of different speech sounds on the emissions of particles from the human respiratory tract.

“The aerosol science community needs to step up and tackle the current challenge presented by COVID-19, and also help better prepare us for inevitable future pandemics,” Ristenpart and colleagues conclude.

A 2005 study that monitored air samples from hospitalized patients infected with SARS during the 2003 outbreak, which was caused by the closest known relative in humans to SARS-CoV-2, found that the patients emitted viable aerosolized virus in the air.

In order to determine the likelihood of becoming infected with the virus that causes COVID-19 through aerosols, we’d have to know the aerosol viral load, as well as the minimum infectious dose for COVID-19 — just because you come in contact with some particles contaminated with SARS-CoV-2 that doesn’t mean you’ll necessarily become infected. Neither of these two important factors has been definitely established.

“Clearly there are many complicated unknowns, which in general have stymied definitive assessment of the role of aerosols in airborne disease transmission. But 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,” the wrote in the journal Aerosol Science and Technology.

This analysis suggests that speech can serve as a plausible transmission mechanism for COVID-19. Although this hypothesis has yet to be verified, it is important to take precautions. Since up to 50% of cases may be asymptomatic, it is thus wise that all of us act as if we might be carriers for the virus and use a face mask when going outside — this is to protect other people rather than protecting ourselves from infection.