SARS-CoV-2 Aerosol Transmission Indoors: A Closer Look at Viral Load, Infectivity, the Effectiveness of Preventive Measures and a Simple Approach for Practical Recommendations

Abstract

There is uncertainty about the viral loads of infectious individuals required to transmit COVID-19 via aerosol. In addition, there is a lack of both quantification of the influencing parameters on airborne transmission and simple-to-use models for assessing the risk of infection in practice, which furthermore quantify the influence of non-medical preventive measures. In this study, a dose-response model was adopted to analyze 25 documented outbreaks at infection rates of 4-100%. We show that infection was only possible if the viral load was higher than 10⁸ viral copies/mL. Based on mathematical simplifications of our approach to predict the probable situational attack rate (PARs) of a group of persons in a room, and valid assumptions, we provide simplified equations to calculate, among others, the maximum possible number of persons and the person-related virus-free air supply flow necessary to keep the number of newly infected persons to less than one. A comparison of different preventive measures revealed that testing contributes the most to the joint protective effect, besides wearing masks and increasing ventilation. In addition, we conclude that absolute volume flow rate or person-related volume flow rate are more intuitive parameters for evaluating ventilation for infection prevention than air exchange rate.

Keywords: SARS-CoV-2; airborne transmission; infection prevention; risk assessment model; simplified approaches.

Figure 1

Relative concentration curve as a function of air change rate and time, based on the steady-state concentration.

Figure 2

Particle emission rates measured by some of the authors, for adults [35,37,38], for adolescents [39] and for children [41].

Figure 3

Virus factor (VF) for different viral loads and particle emission rates with N₀ = 100 viral copies. The attack rates found in the investigated outbreaks are shown with the different colors; the markers show the amount of virus factor at assumed mean particle emission rate of the related activity according to Figure 2.

Figure 4

Virus factor (VF) for different viral loads and particle emission rates with N₀ = 300 viral copies. The attack rates found in the investigated outbreaks are shown with the different colors; the markers show the amount of virus factor at assumed mean particle emission rate of the related activity according to Figure 2.

Figure 5

Specific volume flow depending on the number of emitted particles and the viral load to limit the number of newly infected persons to one; N₀ = 100 viral copies, f_M = 1, Q_b,in = Q_b,ex = 0.54 m³/h.

Figure 6

Comparison of the risk factor x_r for different everyday life situations with a 0.5 h stay in a supermarket, wearing a mask as reference.

Figure 7

Virus factor for different viral loads and critical doses with a particle emission rate of 100 P/s, (left) and 1000 P/s (right); the attack rates found in the investigated outbreaks are shown with different colors.

Figure 8

Viral emission for different viral loads and particle emission rates with N₀ = 67 viral copies; the attack rates found in the investigated outbreaks are shown with different colors.

Figure 9

Specific volume flow depending on the number of emitted, particles and the viral load to limit the number of newly infected persons to one; N₀ = 67 viral copies, f_M = 1, Q_b,in = Q_b,e = 0.54 m³/h.

Figure 10

Influence of different preventive measures on the risk of an outbreak. The red bar represents the viral load and the resulting risk factor (Equation (37)). The blue bars illustrate different combinations of preventive measures in the form of a risk reduction factor (also according to Equation (37)).