Implications of higher viral load in new SARS-CoV-2 variants on airborne transmission

When the coronavirus disease 2019 (COVID-19) pandemic first arose, many governments guided citizens on how to reduce transmission. This advice commonly consisted of telling individuals to wash their hands, social distance, and wear masks. Providing proper ventilation was conspicuously missing from early advice, and while it is now more regularly recommended, this may have increased early transmission rates. Researchers have examined the implications of the increased airborne transmission from new variants.

Study: Increased airborne transmission of COVID-19 with new variants. Implications for health policies. Image Credit: Evgenia.B/Shutterstock

A preprint version of the study is available on the medRxiv* server, while the article undergoes peer review.

The study

The researchers determined that to build a probabilistic infection model, they needed to know the production rate of quanta by an infector, defined by per unit time and per infector. The production rate of quanta can be calculated using the viral load in respiratory fluid, the factor of proportionality between viral content and quanta, the pulmonary exhaled volume rate, size distribution of droplet concentration, and the microbiological characteristics of the variant.

The scientists proposed using different models of transmission for different situations. The homogeneous transmission model assumes no spatial gradient of risk in a space where both infected individuals and susceptible individuals evolve or stay in place. It essentially assumes that infectious molecules in the room are equally distributed. This model is appropriate for situations where ventilation is provided using methods designed to promote high jet induction, leading to a forced convection state. It can also happen when natural convection, individuals moving throughout the room, and doors opening and closing lead to a well-mixed room.

The second model of transmission is called inhomogeneous transmission and occurs when mechanical ventilation due to factors such as air conditioning leads to recirculation of the air. These models typically rely on computational fluid dynamic calculations of airflow stream, with micro-particle behavior estimated using various approaches.

Generally, when COVID-19 is transmitted between individuals at close range, the transmission is airborne. The closer to the expelled infectious particles a susceptible individual is, the higher the viral load they can receive, especially compared to the ambient indoor air. Distance is not the only key factor. However, the amount of time a susceptible individual is exposed to infectious particles in the air can also significantly affect the risk of infection. Generally, larger microdroplets are less worrying than smaller microdroplets, as they are only likely to pass on the infection at very small distances.

The researchers also examined the effects of different ventilation techniques and air quality on transmission. This is split into three rough categories.

1. General ventilation, where the whole volume of air is treated by introducing fresh air from outdoors – either naturally through voluntary vents, open windows, and doors, or ventilation networks throughout the building.
2. Displacement ventilation is more suitable for larger polluted volumes of air. It generates a stratification of the environment by inducing natural convection using fresh air that is colder than the ambient air, which is introduced at a low velocity near the floor.
3. Finally, there is personal ventilation, which is a local technique that treats the microclimate around an occupant in a fixed position. The scientists do not recommend a particular type of ventilation. Instead, they suggest that individuals select a specific type of ventilation that successfully keeps CO2 concentration below certain thresholds and is appropriate for their current situation.

The scientists then examine specific examples, including lecture rooms, school classrooms, and a restaurant. One lecture room, URL5, relied on air intake vents installed in window vents. These were very inefficient and regularly resulted in the CO₂ concentration rising above given thresholds.

The URL20, the second lecture room, was more modern and fitted with a dynamic two-way ventilation system. Even while off, the CO2 concentration was significantly lower than in URL5. The schoolroom examined has a low ventilation rate but not nearly enough to satisfy local regulations. The restaurant was exposed to the wind, which caused significant variation in ventilation values. The restaurant did not keep the CO₂ concentration below local regulations when there was no wind.

The conclusion

The authors highlight that ventilation is one of the most overlooked factors in reducing the rates of COVID-19 transmission. While many countries have updated their guidance and suggested maintaining sufficient airflow, and have started providing public buildings with new ventilation, this is not enough to prevent transmission.

They also point out that new variants are significantly more likely to be transmitted through airborne infection. They argue for new investment into ventilation methods to lower transmission further. This information could be valuable for healthcare workers and possibly inform public health policy.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.