Influence of Possible Natural and Artificial Collective Immunity on New COVID-19 Pandemic Waves in Ukraine and Israel

Background and objectives: The percentage of vaccinated people in Ukraine and Israel extensively varies. Based on this large difference, the influence of possible collective immunity on the COVID-19 pandemic dynamics in summer 2021 was evaluated. Methods: To clarify the presence of a natural collective immunity, the visible and actual characteristics the COVID-19 epidemic in Ukraine were estimated based on the number of laboratory-confirmed cases (accumulated in May and June 2021), using a generalized SIR-model and parameter identification procedure, and considering the difference between registered and real number of cases. Results: The calculated optimal value of the visibility coefficient shows that most Ukrainians have already been infected with coronavirus, some of whom have been infected more than once. This suggests that Ukrainians have probably achieved a natural collective immunity. Despite the large percentage of fully vaccinated people in Israel (approximately 60%), the emergence of a new epidemic wave after June 15, 2021 was not prevented, and the number of deaths increased after July 5, 2021. A new wave of the pandemic in Ukraine after July 10, 2021 is characterized by a smaller daily number of new COVID-19 cases per capita and new deaths per capita, despite having a much lower number of vaccinated people than in Israel. This can be explained by a much lower level of testing (many cases in Ukraine remain undetected) and possibly by the probable natural immunity of Ukrainians. Conclusions: High levels of vaccination and natural collective immunity are unlikely to prevent new waves of the COVID-19 pandemic caused by mutated coronavirus strains.