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
Authors: Kriegel Martin , Hartmann Anne , Buchholz Udo , Seifried Janna , Baumgarte Sigrid , Gastmeier Petra 
Published: November 2021

Currently, airborne transmission is seen as the most important transmission path for SARS-CoV-2. In this investigation, a classic dose-response model is used on the one hand to find out retrospectively the probable viral load of the infectious source patient at the time of transmission in 25 documented outbreaks. We showed that an infection due to airborne transmission at a distance from the infectious person was probably only possible in the 25 outbreaks examined, with attack rates of 4-100%, if the viral load had been higher than 1E+08 viral copies/ml. This demonstrates that the viral load estimated from the swab might overestimate a person’s infectivity via aerosol, because a person is generally considered infectious, independent of the transmission way, when the viral load from the swab is 1E+06 viral copies/ml.

On the other hand, a possible approach is presented to predict the probable situational Attack Rate (PARs) of a group of persons in a room through aerosol particles emitted by an infectious source patient. Four main categories of influence on the risk of infection are formed: First the emitted viruses, depending on the viral load and the amount of respiratory particles, and necessary number of reproducible viruses for infection, second the room-specific data and duration of stay of the group of people, third the activity of the exposed persons, and fourth the effect of personal protection (e.g. wearing masks from infectious and/or susceptible person).

Furthermore, a simplified method is presented to calculate either the maximum possible number of persons in a room, so that probably a maximum of one person becomes infected when an infectious person is in the room, or the PARs,simplefor a given number of persons, ventilation rate and time of occupancy. We additionally show, taking into account organizational preventive measures, which person-related virus-free supply air flow rates are necessary to keep the number of newly infected persons to less than 1. The simple approach makes it easy to derive preventive organizational and ventilation measures. Our results show that the volume flow rate or a person-related flow rate is a much more effective parameter to evaluate ventilation for infection prevention than the air change rate. We suggest to monitor the CO2concentration as an easy to implement and valid measurement system for indoor spaces.

Finally, we show that of the three measures, besides of wearing masks and increasing ventilation, testing contributes the most to the joint protective effect. This corresponds to the classic approach to implement protection concepts: preventing the source from entering the room and emitting viruses at all. In summary, a layered approach of different measures is recommended to mutually compensate for possible failures of any one measure (e.g. incorrect execution of tests, incorrect fit of masks or irregular window opening), to increase the degree of protection and thus reduce the risk of transmission of SARS-CoV-2.