Mechanistic transmission modeling of COVID-19 on the Diamond Princess cruise ship demonstrates the importance of aerosol transmission

Abstract

Several lines of existing evidence support the possibility of airborne transmission of coronavirus disease 2019 (COVID-19). However, quantitative information on the relative importance of transmission pathways of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) remains limited. To evaluate the relative importance of multiple transmission routes for SARS-CoV-2, we developed a modeling framework and leveraged detailed information available from the Diamond Princess cruise ship outbreak that occurred in early 2020. We modeled 21,600 scenarios to generate a matrix of solutions across a full range of assumptions for eight unknown or uncertain epidemic and mechanistic transmission factors. A total of 132 model iterations met acceptability criteria (R² > 0.95 for modeled vs. reported cumulative daily cases and R² > 0 for daily cases). Analyzing only these successful model iterations quantifies the likely contributions of each defined mode of transmission. Mean estimates of the contributions of short-range, long-range, and fomite transmission modes to infected cases across the entire simulation period were 35%, 35%, and 30%, respectively. Mean estimates of the contributions of larger respiratory droplets and smaller respiratory aerosols were 41% and 59%, respectively. Our results demonstrate that aerosol inhalation was likely the dominant contributor to COVID-19 transmission among the passengers, even considering a conservative assumption of high ventilation rates and no air recirculation conditions for the cruise ship. Moreover, close-range and long-range transmission likely contributed similarly to disease progression aboard the ship, with fomite transmission playing a smaller role. The passenger quarantine also affected the importance of each mode, demonstrating the impacts of the interventions.

Keywords: COVID-19; Diamond Princess Cruise Ship; aerosol transmission; built environment; transmission risk model.

Fig. 1.

Reported (actual) and modeled (predicted) cumulative COVID-19 cases aboard the Diamond Princess cruise ship from January 20 to February 24, 2020. Modeled cases are from 132 model scenarios that met acceptable criteria (R² > 0.95 for cumulative daily cases and R² > 0 for daily cases).

Fig. 2.

Estimates of the contributions of transmission modes (A) and viral sources (B) to infected cases aboard the Diamond Princess cruise ship over the entirety of the simulation period.

Fig. 3.

Mean (SD) estimates of (A) the proportion of cases and (B) the effective reproduction number before and after passenger quarantine.

Fig. 4.

Estimates of the contribution of multiple transmission modes to infected cases aboard the Diamond Princess cruise ship over the entirety of the simulation period as well as before and after quarantine measures.

Fig. 5.

Estimates of the contribution of droplets and aerosols to infected cases aboard the Diamond Princess cruise ship over the entirety of the simulation period as well as before and after quarantine measures.