In a recent study posted to bioRxiv*, researchers showed that some severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants of concern (VOCs) exhibit greater surface and aerosol stability than the ancestral strain (WA1).
Several new lineages and variants of SARS-CoV-2 have appeared since the emergence of the lineage A strain. SARS-CoV-2 primarily spreads through the air; fomite and contact transmission routes have also been implicated. The World Health Organization (WHO) has classified five SARS-CoV-2 variants (Alpha, Beta Gamma, Delta, and Omicron) as VOCs, which exhibit higher transmissibility than ancestral SARS-CoV-2.
About the study
In the present study, researchers evaluated whether the increased transmissibility of SARS-CoV-2 VOCs occurred due to changes in surface and aerosol stability. SARS-CoV-2 variant stability was assessed in aerosols and on polyethylene surfaces. Aerosols (less than 5 μm in size) of SARS-CoV-2 variants of 105.75 to 106 median tissue culture infectious dose (TCID50)/ml were produced using a Collison nebulizer.
The inoculum was fed into a rotating Goldberg drum to generate the aerosolized environment. Surface stability on polyethylene was measured by applying a 50 μl solution containing approximately 105 TCID50 of the virus. The exponential decay rates of SARS-CoV-2 were compared at zero-, three-, and eight-hour time points.
Single-run experiments (zero to three hours and zero to eight hours) were performed in triplicates, and viral samples were collected at the start and end points. Reverse-transcription polymerase chain reaction (RT-PCR) tests were performed on air specimens against the SARS-CoV-2 envelope (E) gene. The air specimens were titrated on Vero E6 cells to determine the concentration of remaining viable SARS-CoV-2 virions.
Viable virions of all SARS-CoV-2 VOCs were recovered from the Goldberg drum. The number of viable virions declined exponentially with time. The half-life of the SARS-CoV-2 WA1 strain in aerosols was 3.24 hours. The B.1, Alpha, and Beta variants had longer half-lives than the ancestral strain: 4.01, 6.06, and 5.03 hours for the B.1, Alpha, and Beta variants, respectively.
In contrast, the Delta variant had a similar half-life (3.16 hours) as WA1, but the Omicron variant (2.11 hours) had a shorter half-life than WA1. Next, the researchers calculated the posterior ratio between variant and WA1 half-lives for each VOC. Ratio estimates indicated that the early evolutionary divergence of the spike protein from the ancestral spike seemingly increased the relative stability.
However, further spike divergence reversed the stability to WA1 levels or even reduced it to lower levels. Next, the stability of VOCs relative to the ancestral strain was measured on polyethylene surfaces. All variants showed exponential decay over time on polyethylene surfaces, as observed with aerosols.
The half-life for the WA1 strain was 4.83 hours. The B.1, Alpha, and Beta variants had marginally longer half-lives of 5.19, 5.13, and 5.66 hours, respectively, than WA1. Like in aerosols, the Delta variant (4.36 hours) exhibited a half-life comparable to WA1, whereas the Omicron variant (3.59 hours) had a shorter half-life.
In summary, the researchers observed a slight increase in SARS-CoV-2 stability in aerosols from the ancestral strain through the Beta variant. Nevertheless, the aerosol stability of SARS-CoV-2 Delta was similar to the ancestral strain, but the Omicron variant had reduced stability. The surface stability measurements yielded a similar pattern, implying that similar factors govern the stability of SARS-CoV-2 on surfaces and in aerosols.
Moreover, the findings suggest that aerosol stability may not be the driving factor for the enhanced transmissibility of SARS-CoV-2 VOCs. The increase in stability for the B.1 lineage and its descendants might have been due to the selection of traits favoring increased transmission. Overall, the heterogeneity in the stability of SARS-CoV-2 VOCs on surfaces or aerosols is unlikely to drive the population-level epidemiology of SARS-CoV-2 variants.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information