Role of fomites in SARS transmission during the largest hospital outbreak in Hong Kong

Abstract

The epidemic of severe acute respiratory syndrome (SARS) had a significant effect on global society in the early 2000s and the potential of its resurgence exists. Studies on the modes of transmission of SARS are limited though a number of outbreak studies have revealed the possible airborne route. To develop more specific and effective control strategies, we conducted a detailed mechanism-based investigation that explored the role of fomite transmission in the well-known Ward 8A outbreak. We considered three hypothetical transmission routes, i.e., the long-range airborne, fomite and combined routes, in 1,744 scenarios with combinations of some important parameters. A multi-agent model was used to predict the infection risk distributions of the three hypothetical routes. Model selection was carried out for different scenarios to compare the distributions of infection risk with that of the reported attack rates and select the hypotheses with the best fitness. Our results reveal that under the assumed conditions, the SARS coronavirus was most possible to have spread via the combined long-range airborne and fomite routes, and that the fomite route played a non-negligible role in the transmission.

Fig 1. The three major transmission routes: Long-range airborne, close contact and fomite.

The person in red is the index patient.

Fig 2. System architecture of the multi-agent model.

Fig 3. Distributions of reported attack rates and predicted infection risk and HCWs routine round patterns.

(A) Reported attack rates distribution [20]. (B) Predicted average infection risk distribution (for 1,000 simulations) via the long-range airborne route at 24:00 on March 12, the end of the exposure period. (C) HCWs routine round Pattern 1. (D) Predicted average infection risk distribution via the fomite route (Pattern 1). (E) HCWs routine round Pattern 3. (F) Predicted average infection risk distribution via the fomite route (Pattern 3). (G) HCWs routine round Pattern 5. (H) Predicted average infection risk distribution via the fomite route (Pattern 5). The largest virus-containing droplet size d_g = 100 μm, dose–response parameters in respiratory tracts η_r = 3.2/mRNA copy and on mucous membranes η_m = 3.2 × 10⁻³/mRNA copy and the viral load coefficient c_L = 10. Bed numbers are marked in black in (C), (E) and (G). Reported attack rates and predicted average infection risks for every inpatient are marked in blue in (A), (B), (D), (F) and (H). The intensity of red shading represents levels of attack rate or infection risk.

Fig 4. Illustration of the hypotheses with the minimum RSS in 1,744 scenarios.

d_g denotes the largest virus-containing droplet size (4 values: 50, 100, 150 and 200 μm); η_rc_L and η_mc_L denote products of viral load coefficient and dose–response parameters in respiratory tracts (26 values, 10⁻¹–10⁴/mRNA copy) and on mucous membranes (26 values, 10⁻⁴–10¹/mRNA copy). (A) d_g = 50 μm; (B) d_g = 100 μm; (C) d_g = 150 μm; (D) d_g = 200 μm. Dots of different colours represent different hypotheses as shown in the legend. Dot diameter is inversely proportion to the value of RSS. The smallest RSS in all scenarios (the biggest dot) was 0.5505, and the small black dots represent scenarios with the minimum RSS at least five times as much as 0.5505.