Human Source Severe Acute Respiratory Syndrome Coronavirus 2 Aerosol Transmission to Remote Sentinel Hamsters

Abstract

Background: Bioaerosol-mediated transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) via building ventilation systems has yet to be convincingly demonstrated. We used the South African Airborne Infections Research (AIR) facility near Pretoria to study human-to-animal (H2A) transmission of SARS-CoV-2 in newly diagnosed patients. While the facility was built to study tuberculosis transmission, this was its first adaptation to study H2A virus transmission.

Methods: Patients with clinically confirmed coronavirus disease 2019 were housed for up to 4 days in in the AIR facility with continuously exhausting patient ward air to hamsters housed in animal exposure rooms. After a 3-week exposure period, animals were held for an additional week to allow for antibody development. Animal sera were analyzed for anti-spike and plaque reduction activities and lung samples for pathology.

Results: Seven patients provided ≥400 in-residence hours over a 17-day period. Pair-housed naive golden Syrian hamsters (n = 216) received continuous exposure to mixed patient ward exhaust. Serum analyses revealed anti-SARS-CoV-2 immunoglobulin G in 58% of animals tested. Plaque reduction assays on 7 high-titer serum samples revealed neutralizing activity.

Conclusions: These results support the concept that viral bioaerosols generated from patients remain infectious over long-distance transport through a building ventilation system. The seroconversion among sentinel animals supports the long-held belief that airborne infections manifest as a stochastic rather than deterministic event that is subject to a threshold dose effect. Further confirmatory studies are necessary to characterize the relationship between the bioaerosol delivered and the infections that result in this controlled H2A transmission model.

Keywords: COVID-19; hamster; infectious aerosols; infectious disease aerobiology; respiratory transmission.

Figure 1.

Floorplan schematic of the South African Airborne Infections Research clinical facility. The facility consists of a 6-bed inpatient ward, toilet facilities, and day room, all serviced by an airtight ventilation system and manifolded to 2 animal rooms housing the sentinel animals. All exhaust from the clinical area is shunted so only a portion (approximately 10% of the total flow) enters the individually ventilated animal caging units, with the remaining 90% flow bypassing the caging but passing through the animal rooms. High-efficiency particulate air–filtered flow is supplied to the patient ward at a rate of approximately 6 air changes per hour (ACH); the manifolded air flow to the individual caging units is approximately 25 ACH. Abbreviations: BMS, building maintenance system; HEPA, high-efficiency particulate air.

Figure 2.

Experimental timeline and time-corrected clinical stay of volunteers with coronavirus disease 2019 (COVID-19) for a human-to-animal severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission study. SARS-CoV-2 RNA positivity upon screening was the criterion for admission. Upon clinic admission, medical history was taken, and residential time within the Airborne Infections Research facility and discharge was also voluntary. Mask and swab samples were measured using the Cepheid platform, and color corresponds to the resulting cycle threshold value range: red, 20–25; orange, 25.2–28.4; yellow, 28.6–35; green, 35.2–45. Age range of participants was 23–51 years; race of all participants was black African; 3 were female and 4 male; 2 of 7 (28%) were human immunodeficiency virus positive; and 4 of 7 (57%) were actively taking antibiotics. Signs and symptoms during primary enrollment included tussis (7/7 [100%]), pyrexia (6/7 [85%]), cephalalgia (5/7 [71%]), myalgia (4/7 [57%]), pharyngitis (3/7 [42%]), dyspnea (1/7 [14%]), and ageusia (1/7 [14%]). Clinical signs of patients were not recorded in a systematic way during their stay in the ward for reporting purposes. Naive hamsters were housed beginning −4 days before initial COVID-19 patient stay, maintained throughout the day/date period of observation, and euthanized 21 days from initial clinic air exposure for blood and tissue collection. Abbreviations: HIV⁺, human immunodeficiency virus positive.

Figure 3.

Anti–severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) blood antibodies in the Airborne Infections Research facility–exposed hamsters. Anti-SARS-CoV-2 spike protein immunoglobulin G (IgG) was detected in the serum of hamsters exposed to air from the clinical area. The concentration of IgG was determined in positive samples and plotted alongside the negative serum. The overall assay limit of detection is plotted on the horizontal dotted line. Nonscaled abscissa represents anonymized individual animal blood samples. Abbreviations: Ab, antibody; Ab⁺, antibody positive; LOD, limit of detection; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

Figure 4.

Neutralizing antibodies for highest anti–severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antibody responders (n = 7). Antibodies capable of specifically neutralizing SARS-CoV-2 were characterized by plaque reduction microneutralization testing with replication competent virus using 7 samples with the highest binding antibody concentrations. Percent neutralization curves for each sample are represented; individual data points not shown.