Analysis of the Development of Trends in the Current Epidemic Situation during Spread of the New SARS-CoV-2 Strains and Factors of their Regional Differentiation

Specific processes of the local epidemic dynamics of COVID-19 are analyzed with a comparison of the qualitative differences between fluctuations in 2020 and 2023. A study using the methods of nonlinear dynamics on the development of epidemic processes in the context of a rapidly changing situation required classification of typical trends and unique situations that sometimes change at an extraordinary rate. A rather sharp change in the local trends is a distinctive feature of the modern pandemic: the effect at attenuation of the primary outbreak of morbidity and a sudden sharp beginning of a new epidemic wave after a long trend of decreasing daily infections. Minimizing the infections did not become a victory over the virus, but created a false illusion of success. The existing experience in constructing predictions based on the models of past epidemic processes did not help when encountering a new evolving virus. The previously obtained understanding of the development and completion of epidemic processes for influenza strains interfered when predicting the scenario for the completion of the distribution of the new infection, which is also associated with the event-based nature of the process and the variety of dynamic situations. The victory over COVID that was declared by many countries in the minimum phase after a peak was premature. New Zealand and Japan, which selected the strategy of strict lockdown measures in 2020 experienced an increase in the cases of infections at the beginning of 2023 due to new strains breaking through vaccine immunity. The pandemics of the “Spanish” flu and “swine” flu respiratory viruses (many individuals made predictions based on this analogy) died out naturally after the passage of two or three waves. At the beginning of 2023 (against the background of a global positive trend), a record increase in the indices of both mortality and daily morbidity due to the emergence of locally circulating “alarming” strains was observed in some countries. The current stage with the isolation of stable regional strains substantiates the classification of a number of differentiated properties of the dynamics of regional epidemic situations. Among the observed effects of the epidemic, extreme phenomena of instantaneous destruction of established regimes (a sharp transition from long-damping oscillations to a new exponential outbreak in the number of infections) upon bifurcation are separately highlighted. The selected variants for the development of epidemic transient oscillatory processes are individually described by equations with a delay for local epidemic trends. Equations for the description of three variants of development of the observed stages of local epidemics are proposed. At this stage, the task of constructing a generalizing predictive model of the pandemic for the description of interconnected regional processes is insoluble. The January 2024 Covid wave of recombinant strains JN, XDD from the descendants of the Pirola branch in a number of countries is second only to the original Omicron strain in the number of infections.