April 25, 2024

New WHO Model Estimates Risk of Airborne Transmission of SARS-CoV-2

A new standardized model for quantifying the risk of airborne transmission of SARS-CoV-2 indoors, including in residential, public, and healthcare settings, is described in a new document published by the World Health Organization (WHO). While SARS-CoV-2 can spread in many ways, including by contact transmission, the new model only addresses short- and long-range airborne or inhalation transmission, which WHO says “can occur when [infectious respiratory particles] have travelled either a short or a long distance [. . .] after emission from an infected person or after resuspension of deposited particles from [a] surface.” In addition, the model’s focus on short-range transmission is limited to particle inhalation and does not address direct deposition transmission, which also occurs at close proximity when larger infectious respiratory particles from an infected person are deposited onto the mucosa of a susceptible person.

The new model’s “multi-box approach” allows it to account separately for short- and long-range contributions; according to WHO, “while airborne transmission involves a continuum of exhaled particles of different sizes, short- and long-range risk assessments require a divided yet complementary approach.”

The WHO document outlines five steps or components that affect the “inhalation transmission mechanism” underlying the model: emission rate, removal rate, exposure, cumulative or absorbed dose, and probability of infection. The document defines the emission rate as the number of virus-laden particles exhaled by an infected person per unit of time, while the removal rate is the number of aerosolized virions removed from the air in a given amount of time. Exposure is affected by factors such as distance from the infected source and the volume of a space. According to the document, the cumulative or absorbed dose refers to the number of infectious particles inhaled and absorbed by a susceptible host during exposure, while the final component, probability of infection, is defined by factors including the pathogen infectious dose, the immunological status of the host, and the transmissibility of a specific SARS-CoV-2 variant.

“The model aims to inform the development of risk-based ventilation recommendations to mitigate the inhalation risk, as well as to provide an agreed foundation for future guidance related to other respiratory and potentially airborne diseases,” the WHO document explains.

WHO’s new model was also used to develop a new online tool called ARIA, short for “Airborne Risk Indoor Assessment,” which is intended to help infection prevention and control specialists, building managers, and the general public estimate the risk of SARS-CoV-2 airborne transmission in various indoor settings. ARIA is also meant to help users make informed decisions regarding risk reduction measures, including increasing ventilation, reducing occupancy, and wearing a mask with a higher filtration efficiency.

The new document, “Indoor Airborne Risk Assessment in the Context of SARS-CoV-2,” is available for download from the WHO website.

Related: The article “A Dashboard for COVID-19 Risk” in the October 2022 Synergist discusses an exposure assessment tool that can be used to calculate the relative risk of transmission of SARS-CoV-2.