Persistence of Severe Acute Respiratory Syndrome Coronavirus 2 in Aerosol Suspensions

We aerosolized severe acute respiratory syndrome coronavirus 2 and determined that its dynamic aerosol efficiency surpassed those of severe acute respiratory syndrome coronavirus and Middle East respiratory syndrome. Although we performed experiment only once across several laboratories, our findings suggest retained infectivity and virion integrity for up to 16 hours in respirable-sized aerosols.

for SARS-CoV (p = 0.02) or MERS-CoV (p = 0.01). Because SARS-CoV was aerosolized into a different chamber/volume than MERS-CoV and SARS-CoV-2, we cannot rule out chamber effects for the difference in Fs between SARS-CoV and SARS-CoV-2. Our comparison of nebulizers showed improved Fs for SARS-CoV-2 with the C6 (p = 0.006) and the AS (p = 0.01) over the C3 but no difference between the C6 and AS (p = 0.46).
Further studies with SARS-CoV-2 at Tulane preliminarily assessed the long-term stability of airborne virus. We used a custom-built rotating (Goldberg) drum to provide an environment in which rotational drum speed overcomes the terminal settling velocity of the 2-3-µm particles, providing a static aerosol suspension of known volume (4)(5)(6). We timed aerosol samples from the drum at 10 min and 30 min and at 2 h, 4 h, and 16 h after initiation of rotation/suspension. The entire drum volume (10.7 L) was evacuated at each sampling interval and represented a discrete aerosol generation event. We quantified virus contents by plaque assay and quantitative reverse transcription PCR (qRT-PCR). We also conducted scanning electron microscopy on the collected aerosol samples as a complimentary qualitative assessment of virion integrity after longer-term aerosol suspension (Appendix). We measured environmental parameters but did not control them during the aerosol suspension experiments. The prevailing ambient environmental conditions were 23°C SD ± 2°C and 53% SD ± 11% relative humidity throughout the aerosol stability experiments. No ultraviolet light source was used within the cavity of the drum during suspensions. After initial generation of viral bioaerosols into the drum reached steady-state concentration, the drum was sealed and maintained as a static aerosol. We conducted all sampling time points once in this set of experiments.
We graphed plaque assay and qRT-PCR results and applied nonlinear least-squares regression analysis single-order decay with no outlier detection, resulting in a poor curve fit, which typically results from a lack of replicate samples. We detected infectious SARS-CoV-2 at all time points during the aerosol suspension stability experiment ( We also performed a qualitative assessment of virion integrity after longer-term aerosol suspension (Appendix). Scanning electron microscopy (SEM) imaging of SARS-CoV-2 revealed virions that were heterogeneous in shape, either ovoid (Appendix Figure, panel A) or spherical (Appendix Figure, panel B). The minor:major axis ratio of oval virions was ≈0.7, which is consistent with prior SEM analyses of SARS-CoV-2 (https://www.flickr.com/photos/niaid/albums/72157712914621487). Airborne SARS-CoV-2 maintained the expected morphologies, size, and aspect ratios up to 16 h. Specifically, virions aged for 10 min (Appendix Figure, panels C, D) or 16 h (Appendix Figures, panels E, F) were similar in shape and general appearance to virions examined in samples of viral inoculum collected before aerosolization, which is consistent with the retention of replication-competence and suggests the potential to be infectious after long-term aging in aerosol suspension.

Conclusions
The comparison of short-term aerosol efficiencies of 3 coronaviruses showed SARS-CoV-2 approximates or exceeds the efficiency estimates of SARS-CoV and MERS-CoV. Some efficiency determinations for SARS-CoV-2 ranged to -5.5 log10 (Figure 1), a full log difference from MERS-CoV. The higher efficiencies across independent laboratories strengthens this observation. These data suggest that SARS-CoV-2 generally maintains infectivity at a respirable particle size over short distances, in contrast to either betacoronavirus. Aerosol suspension results suggest that SARS-CoV-2 persists longer than would be expected when generated as this size particle (2-µm mass median aerodynamic diameter). This finding is notable because decay and loss in the infectious fraction of airborne virus would be expected on the basis of prior susceptibility studies with other environmentally hardy viruses, such as monkeypox virus (5). A recent study (6) showing only a slight reduction of infectivity in aerosol suspensions with approximately similar particle sizes also suggested minimal effects on SARS-CoV-2 airborne degradation.
Collectively, these preliminary data suggest that SARS-CoV-2 is resilient in aerosol form, agreeing with conclusions reached in earlier studies of aerosol fitness (6). A limitation of our data is that we report 2170 Emerging Infectious Diseases • www.cdc.gov/eid •Vol. 26, No. 9, September 2020 only 1 measurement of the 16-h time point; these findings must be replicated before definitive conclusions are reached. However, our results indicate that aerosol transmission of SARS-CoV-2 may be a more important exposure transmission pathway than previously considered (7). Our approach of quantitative measurement of infectivity of viral airborne efficiency augmented by assessment of virion morphology suggests that SARS-CoV-2 may be viable as an airborne pathogen. Humans produce aerosols continuously through normal respiration (8). Aerosol production increases during respiratory illnesses (9,10) and during louder-than-normal oration (11). A fraction of naturally generated aerosols falls within the size distribution used in our experimental studies (<5 µm), which leads us to conclude that SARS-CoV-2-infected persons may produce viral bioaerosols that remain infectious for long periods after production through human shedding and airborne transport. Accordingly, our study results provide a preliminary basis for broader recognition of the unique aerobiology of SARS-CoV-2, which might lead to tractable solutions and prevention interventions.