In a recent study, Japanese researchers report that the new severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant, Omicron, has higher environmental stability than previous variants.
The study compared the SARS-CoV-2 Wuhan strain and variants of concern (VOCs) - Alpha, Beta, Delta, and Omicron variants for their survivability and infectivity.
Using constructed human skin models, and plastic substrates, the researchers evaluated the environmental stability of the variants.
Understanding the pathogenesis of the SARS-CoV-2 and its stability in the environment is critical in mitigating the ongoing pandemic that has caused debilitating damages globally.
From the initial days, the SARS-CoV-2 ancestral virus has exhibited high transmissibility. With the emergence of many variants, the virus has gained higher transmissibility as evidenced by the number of infection cases despite vaccines and lockdowns the world over.
Several factors are attributed to the increased infectivity and transmissibility of the SARS-CoV-2, such as increased viral load shed from infected individuals, prolonged viral shedding period, decrease in the minimum viral load required to establish infection, changes in the infection target site, and increased environmental stability.
However, to date, no study directly compares the stability of the ancestral virus and its variants of concern in detail and reports their environmental stability. There are previous studies that have reported that the Alpha (Pango lineage: B.1.1.7) and Beta (Pango lineage: B.1.351) have similar stability. Some studies have compared the stability of the SARS-CoV-2 with that of the severe acute respiratory syndrome coronavirus (SARS-CoV-1) and the influenza virus.
The present study precisely evaluated the differences in viral stability of the Wuhan strain (Pango lineage: A) and all the VOCs, including the Omicron (Pango lineage: B.1.1.529) and Delta (Pango lineage: B.1.617.2) variant. They also analyzed the disinfection efficacy between the Wuhan strain and all VOCs.
All viruses ( Wuhan strain (Pango lineage: A, hCoV-19/Japan/TY/WK-521/2019), Alpha variant (Pango lineage: B.1.1.7, hCoV-19/Japan/QK002/2020), Beta variant (Pango lineage: B.1.351, hCoV-19/Japan/TY8-612/2021), Gamma variant (Pango lineage: P.1, hCoV-19/Japan/TY7-501/2021), Delta variant (Pango lineage: B.1.617.2, hCoV-19/Japan/TY11-927/2021), and Omicron variant (Pango lineage: B.1.1.529, hCoV-19/Japan/TY38-873/2021)) in the study were provided by the National Institute of Infectious Diseases (Tokyo, Japan). The researchers used the VeroE6/TMPRSS2 cells for culturing the viruses.
The researchers evaluated the virus stability on plastic (polystyrene plate) and human skin surfaces. Using the human skin collected from forensic autopsy specimens, the researchers developed an ex vivo model.
After applying the virus on the plastic/skin surface, they tested for the survivability of the virus. They evaluated the stability of different viruses on the surface of the human skin model, and also found the effectiveness of different disinfectants against viruses on the human skin. The alcohol-based disinfectants, ethanol, and isopropanol were used in the study.
The researchers defined the detection limit for the titer of the virus remaining on the surface to be 100.5 TCID50, and the survival time as the time until the virus on the surface was no longer detected.
On the plastic surface, the researchers analyzed the survival times of the Wuhan strain, Alpha variant, Beta variant, Gamma variant, Delta variant, and Omicron variant to be 56.0 h, 191.3 h, 156.6 h, 59.3 h, 114.0 h, and 193.5 h, respectively.
While on the human skin surface, they analyzed the survival times of the Wuhan strain, Alpha variant, Beta variant, Gamma variant, Delta variant, and Omicron variant to be 8.6 h, 19.6 h, 19.1 h, 11.0 h, 16.8 h, and 21.1 h, respectively; with the Omicron variant having the longest survival time. The researchers reported that the half-life values had the same tendency as the survival time.
The disinfectants were effective against all the viruses within 15s. However, the VOCs were slightly more resistant than the Wuhan strain. On human skin, an ex vivo evaluation showed complete inactivation of all the viruses with exposure to 35% ethanol within 15s. Therefore, the researchers highly recommend the current protocol of hand hygiene practices to continue for infection control as advised by the World Health Organization.
Thus, this study showed that on plastic and skin surfaces, the VOCs exhibited more than two-fold longer survival times than those of the Wuhan strain and remained infective on the skin surfaces for more than 16h.
The study does not address the reasons for the observed higher environmental stability of the variants.
Further research using recombinant viruses may identify the factors behind this, the researchers observed. Importantly, the evaluated survival time and the half-life in this study are dependent on the external environment and the composition of the body fluid.
In this study, the virus was ultracentrifuged and dispersed in PBS (phosphate buffer saline) solvent. Therefore, it is subject to change according to the number of variables in the environment.
Moreover, the relationship between the virus load on the surface and the risk of transmission is unclear.
The information on the stability of the virus and its variants responsible for the ongoing pandemic is crucial for infection control and mitigation. This study investigated the environmental stability of the SARS-CoV-2 and its variants of concern. It showed that the Omicron variant has the highest environmental stability among VOCs; suggestive of how Omicron has rapidly replaced the Delta variant and ferociously spread across the globe.
Furthermore, these findings will contribute greatly to elucidating the mechanism of VOC spread with the addition of genetic analysis, the researchers conclude.