The vaccination threshold for SARS-CoV-2 depends on the indoor setting and room ventilation

Abstract

Background: Effective vaccines are now available for SARS-CoV-2 in the 2nd year of the COVID-19 pandemic, but there remains significant uncertainty surrounding the necessary vaccination rate to safely lift occupancy controls in public buildings and return to pre-pandemic norms. The aim of this paper is to estimate setting-specific vaccination thresholds for SARS-CoV-2 to prevent sustained community transmission using classical principles of airborne contagion modeling. We calculated the airborne infection risk in three settings, a classroom, prison cell block, and restaurant, at typical ventilation rates, and then the expected number of infections resulting from this risk at varying percentages of occupant immunity.

Results: We estimate the setting-specific immunity threshold for control of wild-type SARS-CoV-2 to range from a low of 40% for a mechanically ventilation classroom to a high of 85% for a naturally ventilated restaurant.

Conclusions: If vaccination rates are limited to a theoretical minimum of approximately two-thirds of the population, enhanced ventilation above minimum standards for acceptable air quality is needed to reduce the frequency and severity of SARS-CoV-2 superspreading events in high-risk indoor environments.

Keywords: Airborne transmission; SARS-CoV-2; Vaccination; Ventilation.

Fig. 1

Surface graphs of R_event for wild-type SARS-CoV-2 as a function of the number of susceptibles and air exchange rate (AER) for the restaurant (A), classroom (B) and prison cell block (C) modeling scenarios. Contour lines connect equal R_event values. The black- and white-filled points along the R_event = 1.0 contour line identify the threshold number of susceptibles for natural ventilation and mechanical ventilation scenarios, respectively, at the intersection of the dashed horizontal and vertical lines. The threshold values are labeled in parenthesis in terms of both the percent susceptible and m² susceptible⁻¹

Fig. 2

Minimum quanta emission rates (ER_q) for R_event ≥ 1.0 for the prison scenario under natural ventilation, mechanical ventilation, and high air quality ventilation conditions as a function of the number of susceptibles. Points #1 and #2 identify the minimum emission rates for high air quality ventilation and natural ventilation at their respective threshold number of susceptibles from Fig. 1C. Point #3 identifies the minimum emission rate for high air quality ventilation at the natural ventilation threshold number of susceptibles, representing both high ventilation and high vaccination. The minimum emission values are labeled in parenthesis, denoting the emission in quanta h⁻¹ and its corresponding percentile in the resting, oral breathing ER_q distribution