The UK, along with many countries across the world, is in lockdown. This is because the Covid-19 virus is deemed to be deadly and highly transmissible. It is a bit of a mystery as to how it is so contagious, but here is the reason.

This question is easy to ask but extremely challenging to answer. When an infected individual reports to a hospital there is no way to assess definitively how they were infected.

The “contact-tracing” performed by epidemiologists carefully tracks who came into “close contact” with a patient under investigation, but it cannot tell you how the virus itself was transferred from the contagious person to those whom they infected.

There is broad agreement in the infectious disease community about possible modes of respiratory virus transmission between humans.

Direct or indirect “contact” modes require a susceptible individual to physically touch themselves with, for example, a virus-contaminated hand;

  • “direct” indicates that person-to-person contact transfers the virus between infected and susceptible hosts (such as by a handshake), while
  • “indirect” implies transmission via an object like a hand-rail or paper tissue that has been contaminated with infectious virus.

In contrast, airborne transmission may occur by two distinct modes and requires 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 µm 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 (<5 µm) to remain airborne for hours.

People have been advised to focus on avoiding contact with coughing and sneezing individuals, as these ‘violent’ expiratory events carry infected droplet sprays for up to 8 metres. Our current 2 metre social distancing rule may not be as effective as we think.

My wife and I walked past two ladies who were walking and talking the other day. As we passed, observing the two metre rule, we could smell perfume quite strongly, and carried on smelling it for twenty paces. I remarked to my wife that this could be a good analogy for the cloud of  respiratory aerosol particles that people exhale when they talk. They expel the particles, and if they are in a room they will remain in the air, much like you can smell a person’s perfume or aftershave, even when they have left the room.

Runners and cyclists, meanwhile, exhale respiratory aerosol particles at an even greater rate due to the increased rate and force of breathing during this type of exercise. They also leave a slipstream of aerosol particles up to 20 metres behind them.

There is strong evidence now that many infected individuals (about 80%) who transmit COVID-19 are either minimally symptomatic or not symptomatic at all. 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 and did not cough or sneeze any more than normal.

Much media attention has quite rightly focused on the possibility of direct and indirect transmission via for example contaminated hands, with public health messages focusing on the importance of washing hands thoroughly and often, and of greeting others without shaking hands. There is no doubt that this has been beneficial.

Aerosolized SARS-CoV-2 remains viable in the air with a half-life of one hour; both aerosol and surface transmission of SARS-CoV-2 occurs, since the virus can remain viable and infectious in aerosols for hours and on surfaces for days.

But if 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. These expiratory particles are typically about 1 micron in diameter, and thus invisible to the naked eye. The particles are sufficiently large 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 (another viral respiratory disease) has established that viable virus can indeed be emitted from an infected individual by breathing or speaking, without coughing or sneezing.

Ordinary speech aerosolizes significant quantities of respiratory particles. A ten-minute conversation with an infected, asymptomatic person talking normally 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. While viral load does play a role in the severity of an infection, studies on other respiratory viruses show that one virus is enough to kickstart an infection.

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 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.

For this reason, social distancing and the wearing of face masks may well be with us for a considerable period of time.