Mathematical Modeling for Delta and Omicron Variant of Sars-Cov-2 Transmission Dynamics in Greece

A compartmental epidemic mathematical model has been developed to analyze the transmission dynamics of Delta and Omicron variant, of SARS-CoV-2, in Greece. The model has been parameterized twice during the 4th and 5th wave of pandemic. The 4th wave refers to the period that the Delta variant was dominant (approximately July to December 2021) and the 5th wave to the period that the Omicron variant was dominant (approximately January to May 2022), according to the official data from the National Public Health Organization (NPHO). Fitting methods were applied to evaluate important parameters related to the transmission of the variants, but also to the social behavior of population during these periods of interest. Mathematical modeling revealed higher transmission and possibility of asymptomatic disease during the Omicron variant period, but decreased rate of hospitalization compared to the Delta period. Also, parameters related to the behavior of the population in Greece were also assessed, in particular regarding mask usage and the observance of social distancing measures. Simulations revealed that over 5,000 deaths would have been avoided, if mask usage and social distancing were 20% more efficient, during a few months of the Delta and Omicron period. Furthermore, the spread of the variants was assessed using viral load data. Data was recorded from PCR tests at 417 Army Equity Fund Hospital (NIMTS), in Athens and the Ct values from 746 patients with COVID-19 were processed, to explain transmission phenomena and disease severity in patients. The period that the Delta variant prevailed in the country, the average Ct value was calculated as 25.19 (range: 12.32-39.29), whereas during the period that the Omicron variant prevailed, the average Ct value was calculated as 28 (range: 14.41-39.36). In conclusion, our experimental study showed that the higher viral load related to the Delta variant may interpret the disease severity, but no correlation was confirmed regarding transmission phenomena. The results of the model, Ct analysis and official data from NPHO are consistent.