Differences in environmental stability among SARS-CoV-2 variants of concern: both omicron BA.1 and BA.2 have higher stability

Abstract

Objectives: The increased infectivity and transmissibility of SARS-CoV-2 variants of concern (VOCs) could cause significant human and economic damage. Hence, understanding their characteristics is crucial to control infection. We evaluated the environmental stability of the Wuhan strain and all VOCs (Alpha, Beta, Gamma, Delta, Omicron BA.1, and Omicron BA.2 variants) on plastic and human skin surfaces and their disinfection efficacy.

Methods: To evaluate environmental stability, residual virus titres on plastic and human skin surfaces were measured over time. Their survival time and half-life were calculated using regression analysis. The effectiveness of ethanol-based disinfectants at different concentrations was determined by in vitro and ex vivo evaluations.

Results: On plastic and skin surfaces, the Alpha, Beta, Delta, and Omicron variants exhibited approximately two-fold longer survival times than the Wuhan strain; the Omicron variants had the longest survival time. The median survival times of the Wuhan strain and the Alpha, Beta, Gamma, Delta, and Omicron (BA.1 and BA.2) variants on human skin surface were 8.6, 19.6, 19.1, 11.0, 16.8, 21.1, and 22.5 h, respectively. The in vitro evaluation showed that the Wuhan strain and the Alpha, Beta, Gamma, Delta, and Omicron (BA.1 and BA.2) variants were completely inactivated within 15 s by 32.5%, 35%, 35%, 32.5%, 35%, 40%, and 40% ethanol, respectively. However, all viruses on human skin were completely inactivated by exposure to 35% ethanol for 15 s.

Conclusions: SARS-CoV-2 VOCs, especially the Omicron variants, have higher environmental stability than the Wuhan strain, increasing their transmission risk and contributing to their spread.

Keywords: Disinfection effectiveness; Environmental stability; SARS-CoV-2; Variants of concern; Virus survival.

Fig. 1

(A, B) Decrease in residual viral titre on plastic (A) and human skin (B) surfaces over time. Each virus (4.7 Log₁₀TCID₅₀; 50% tissue culture infectious dose) was mixed with 2 μL of phosphate-buffered saline and applied to each surface, which was incubated in a controlled environment (temperature: 25°C, humidity: 45%–55%) for 0–144 h. Viruses on the surface were then recovered in 1 mL of medium and titrated to calculate the titres of the remaining virus. For each condition, three independent experiments were performed, and the results are expressed as the mean ± standard error of the mean. The bars for data below the detection limit were omitted. Dotted horizontal lines represent detection limit titres. The values of residual viral titre on surfaces were evaluated with Student's t-test, and the magnitudes with P < 0.05 were considered significant. ∗P < 0.05 (versus the Wuhan strain). ^†P < 0.01 (versus the Wuhan strain). (C, D) Stability of viruses on the plastic surface (C) and the human skin surface (D). The elapsed time was defined as an explanatory variable (X-axis), and the log virus titre was defined as an explained variable (Y-axis). Least-squares linear-regression analysis was performed with a logarithmic link function for each virus to generate a regression curve. The upper and lower confidence limits are represented by dotted curves. The dotted horizontal lines represent the detection limit titres. The data shown are expressed as the mean ± standard error of the mean for three independent experiments.

Fig. 2

(A) Survival times of the various viruses on a plastic surface. (B) Survival times of the various viruses on the surface of human skin. (C) Half-lives of the various viruses on a plastic surface. (D) Half-lives of the various viruses on the surface of human skin. Survival time is defined as the time until the virus can no longer be detected on the surface. All half-lives in the graphs refer to a condition when 2.0 and 3.0 Log₁₀TCID₅₀ (50% tissue culture infectious dose) of virus particles remained on the surface. Data are expressed as the median ±95% confidence interval.

Fig. 3

In vitro evaluation of disinfectant effectiveness. The ethanol concentration was defined as an explanatory variable (X-axis), and the log virus titre concentration was defined as an explained variable (Y-axis). The results of the log virus titre concentration are expressed as the mean ± standard error. Nonlinear regression analyses were conducted using a 4-parameter logistic model. The upper and lower confidence limits of the regression curves are omitted. The ethanol concentration required to achieve a logarithmic decrease of 3.5 in virus titre concentration in a 15 s disinfection reaction (notated as the required ethanol concentration) was defined as the X values when the Y values of the regression curves were 1.0.