Differences in infectivity and pathogenicity between Delta and Omicron strains of SARS-CoV-2 can be explained by Gibbs energies of binding and growth

During the COVID-19 pandemic, biothermodynamics has given its contribution to characterization of viruses and research on energetics of processes performed by viruses [1-4]. Thermodynamic properties represent the driving force for processes performed by viruses and hence are an important element in predictive mechanistic models of virus-host interactions [2,3]. In this research, empirical formulas have been reported of the Delta and Omicron strains of SARS-CoV-2. The empirical formula of the Delta strain virion was found to be CH O N P S . The empirical formula of the Omicron strain virion was found to be CH O N P S . Based on the empirical formulas, standard thermodynamic properties of formation and growth have been calculated and reported for the Delta and Omicron strains. Moreover, standard thermodynamic properties of binding have been reported for Wild type (Hu-1), Alpha, Beta, Gamma, Delta and Omicron strains. For all the strains, binding phenomenological coefficients and antigen-receptor (SGP-ACE2) binding rates have been determined and compared, which are proportional to infectivity. The results show that infectivity of the Omicron strain is 50% greater than that of the Delta strain. The increased infectivity was explained in this paper using Gibbs energy of binding. However, no indications exist for decreased pathogenicity of the Omicron strain. Pathogenicity is proportional to the virus multiplication rate, while Gibbs energies of multiplication are very similar for the Delta and Omicron strains. Thus, multiplication rate and pathogenicity are similar for the Delta and Omicron strains. The lower number of severe cases caused by the Omicron strain can be explained by increased number of immunized people. Immunization does not influence the possibility of occurrence of infection, but influences the rate of immune response, which is much more efficient in immunized people. This leads to prevention of more severe Omicron infection cases.