A question of time? The dynamics of airborne COVID-19 virus infectivity
In January, researchers at the University of Bristol, part-funded by PROTECT, published a pre-print paper on how the infectivity of airborne particles containing SARS-CoV-2 (the virus that causes COVID-19) changes over time in different environments. Using a new laboratory technique known as CELEBS (‘Controlled Electrodynamic Levitation and Extraction of Bioaerosols onto a Substrate’), the team explored the impact of rapid changes in particle composition and moisture content on virus survival.
People infected with SARS-CoV-2 exhale particles containing the virus when they breathe, talk, sing or cough. These particles cover a large size range, from under one micrometre in diameter to over 100 micrometres.
Particles larger than 100 micrometres (a little larger than the diameter of a human hair; sometimes referred to as droplets) settle over short distances and may contaminate surfaces. People don’t produce many of these large particles unless they cough or sneeze.
Most of the particles people exhale are smaller than 100 micrometres and are referred to as aerosols. These can be inhaled, and can also evaporate to become smaller once they leave the mouth and nose. The smallest particles can remain in the air, drifting around for minutes or even hours, exposing uninfected individuals and leading to virus transmission.
The amount of virus that an infected person exhales is dependent on a number of factors, including whether they are breathing or vocalizing (speaking or singing), how loudly they are vocalizing, and the viral load in the respiratory fluid forming the aerosol within the nose, throat or lung.
In an outdoor environment, inhalable particles are rapidly dispersed and significantly diluted by the fresh air, reducing the risk of transmission. However, in an indoor space, these particles may take a long time to be displaced from a room. If ventilation is poor, they can accumulate over time, leading to transmission both between people in close proximity and over longer distances across the room.
To reduce the risk of transmission, the dose of virus that people inhale must be reduced. This can be done using measures such as face coverings and effective ventilation (whether natural or mechanical). Physical distancing can also reduce exposure to both particles larger than 100 micrometres and the higher concentration of smaller particles that are present closer to an infected person.
The impact of time and humidity
One further piece of the jigsaw puzzle required to understand transmission of SARS-CoV-2 through inhaled particles is the impact of time and environmental conditions on the infectivity of the virus while airborne.
Previous studies (van Doremalen et al., Smither et al. and Schuit et al.) have reported that SARS-CoV-2 can remain infectious for many hours when airborne, decaying to half of the starting infectious virus over a period of one to two hours. These studies, performed by introducing a cloud of virus-carrying aerosols into a large rotating drum to keep them suspended in air, have examined changes in infectivity over timescales from 20 minutes to many hours. Regardless of time, these studies had also shown little impact on infectivity from changes in the relative humidity of the air in the drum.
However, humidity does have an important effect on respiratory particles. Immediately on exhalation, particles are transferred from a humid environment with high carbon dioxide concentration inside the body into the drier indoor environment, with much lower carbon dioxide concentration. The particles must adjust rapidly to this change in conditions – losing moisture causes them to evaporate and they also rapidly lose dissolved carbon dioxide.
Using the CELEBS technique, around 10 particles containing the virus are levitated in an electrodynamic field for any chosen period of time, from seconds to hours. The technique allowed us to observe changes that could not be seen using the rotating drum method. At humidity levels commonly experienced in buildings (less than 50%), more than half of the infectious virus is lost almost instantaneously (in under 10 seconds) as the particles lose water. At high humidity (90%) the loss is slower but becomes significant after 10 minutes, by which point about 80% of infectious virus is lost. These effects were consistent across three different SARS-CoV-2 variants.
Laboratory experiments using CELEBS can provide insights into the mechanisms that drive changes in airborne infectivity of SARS-CoV-2 in the real world. Although the compositions of the liquid aerosols containing the virus in our experiments could not yet replicate the complexity of real respiratory particles, they were consistent with those used in previous rotating drum studies. Together, these studies suggest that any rapid initial decline in infectivity driven by rapid changes in moisture and carbon dioxide content is likely followed by a slower loss of remaining infectivity over a much longer time.
So, does this decline in infectivity mean that longer range airborne transmission is unlikely, and that we only really need to worry if we are close to the person exhaling the virus? Unfortunately, it is not that simple.
Although a 50-90% reduction sounds large, it is only a drop of one order of magnitude. For context, there is around a 1,000 times variation in the amount of aerosol that different people produce, and their viral load at peak infectiousness can vary by over 10,000 times.
Even with a reduction in infectivity, there are still occasions when there is sufficient infectious virus in the air at considerable distance from the infected person that someone can inhale and become infected. We know from epidemiological data that airborne transmission is not rare and can lead to superspreading events where one person infects many others at the same time.
Our new study adds an extra piece to the complicated jigsaw puzzle of transmission, but it doesn’t change the key messages. Transmission frequently occurs through inhalation of the virus, and therefore measures such as face coverings and ventilation – alongside vaccination and testing – remain critical to managing COVID-19, especially as we move from pandemic to endemic.