Omicron was found to have higher environmental stability among SARS-CoV-2 variants
In a recent study, Japanese researchers report that the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant, Omicron, has higher environmental stability than previous variants.
The study, published on the preprint server bioRxiv*, compared the SARS-CoV-2 Wuhan strain and variants of concern (COV) – Alpha, Beta, Delta and Omicron variants for their survivability and infectivity.
Using constructed human skin models and plastic substrates, researchers assessed the environmental stability of the variants
Understanding the pathogenesis of SARS-CoV-2 and its stability in the environment is critical to mitigating the ongoing pandemic that has caused debilitating damage globally.
From the earliest days, the ancestral SARS-CoV-2 virus showed high transmissibility. With the emergence of many variants, the virus has acquired higher transmissibility, as evidenced by the number of infection cases despite vaccines and lockdowns worldwide.
Several factors are attributed to the increased infectivity and transmissibility of SARS-CoV-2, such as increased viral load excreted by infected individuals, prolonged period of viral shedding, decreased trough viral load required to establish infection, changes in the target site of infection, and increased environmental stability.
However, to date, no studies directly compare in detail the stability of the ancestral virus and its variants of concern and report on their environmental stability. There are previous studies that reported that Alpha (Pango line: B.1.1.7) and Beta (Pango line: B.1.351) have similar stability. Some studies have compared the stability of SARS-CoV-2 with that of severe acute respiratory syndrome coronavirus (SARS-CoV-1) and influenza virus.
The present study precisely evaluated the differences in viral stability of the Wuhan strain (Pango lineage: A) and all VOCs, including Omicron (Pango lineage: B.1.1.529) and Delta (Pango lineage: B.1.617 .2) a variation. They also analyzed the effectiveness of disinfection 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 (lineage Pango: B.1.617.2, hCoV-19/Japan/TY11-927/2021) and the 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 VeroE6/TMPRSS2 cells to grow the viruses.
The researchers evaluated the stability of the virus on plastic surfaces (polystyrene plate) and human skin. Using human skin taken from forensic autopsy specimens, researchers developed a ex-vivo model.
After applying the virus to the plastic/skin surface, they tested the survivability of the virus. They evaluated the stability of different viruses on the surface of the human skin model and also discovered the effectiveness of different disinfectants against viruses on human skin. Alcohol-based sanitizers, ethanol, and isopropanol were used in the study.
The researchers defined the detection limit for the titer of virus remaining on the surface as 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 at 56.0 h, 191 .3 hrs, 156.6 hrs, 59.3 hrs, 114.0 hrs and 193.5 hrs, respectively.
On the surface of human skin, they analyzed the survival times of Wuhan strain, Alpha variant, Beta variant, Gamma variant, Delta variant and Omicron variant at 8.6 h, 7.6 p.m., 7.1 p.m., 11.0 a.m., 4.8 p.m., and 9.1 p.m., respectively; with the Omicron variant having the longest survival time. The researchers reported that the half-life values had the same trend as the survival time.
The disinfectants were effective against all viruses within 15 seconds. However, VOCs were slightly more resistant than the Wuhan strain. On human skin, a ex-vivo evaluation showed complete inactivation of all viruses with exposure to 35% ethanol within 15 seconds. Therefore, the researchers strongly recommend continuing the current protocol of hand hygiene practices for infection control as advised by the World Health Organization.
Thus, this study showed that on plastic and skin surfaces, VOCs had survival times more than twice as long as those of the Wuhan strain and remained infectious on skin surfaces for more than 16 hours.
The study does not address the reasons for the observed greater environmental stability of the variants.
Further research using recombinant viruses could identify the factors behind this, the researchers observed. Importantly, the assessed survival time and half-life in this study depend on the external environment and body fluid composition.
In this study, the virus was ultracentrifuged and dispersed in PBS (phosphate buffer saline) solvent. Therefore, it is subject to change depending on the number of variables in the environment.
In addition, the relationship between surface viral load and transmission risk is unclear.
Therefore, it might be reasonable to interpret the survival time value in this study as a reference value,” the researchers observed in the paper.
Information on the stability of the virus and its variants responsible for the ongoing pandemic is crucial for infection control and mitigation. This study examined the environmental stability of SARS-CoV-2 and its variants of concern. It showed that the Omicron variant has the highest environmental stability among VOCs; evocative of how Omicron quickly replaced the Delta variant and ferociously spread across the world.
Moreover, these findings will go a long way towards elucidating the mechanism of spread of VOCs with the addition of genetic analyses, the researchers conclude.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be considered conclusive, guide clinical practice/health-related behaviors, or treated as established information.