MANIFESTATIONS OF THE COVID-19 EPIDEMIC: INSIGHTS FROM A CONTINUOUS RETROSPECTIVE OBSERVATIONAL STUDY

УДК 616.988:578.843.1:615.371

Abstract

Introduction. Observational evaluations of vaccination effectiveness, particularly cohort studies, are hindered by differences in key characteristics between vaccinated and unvaccinated cohorts, leading to systematic errors. Hence, careful design and consideration of the specific epidemic process in a given setting are essential in assessing vaccination effectiveness. Aim. The study aimed to establish the manifestations of the COVID-19 epidemic process necessary for subsequent evaluation of COVID-19 vaccination efficacy in real-life settings. Materials and Methods. A continuous observational retrospective study was conducted to investigate the structure, dynamics, and COVID-19 morbidity rates of the adult population of Minsk, served by one of the polyclinics, from April 2020 to March 2023, using outpatient medical records (Form 025/u-07). Results. Seven waves of COVID-19 incidence, varying in level and circulating virus variants, were identified over 5-6 month periods. The frequency of recurrent COVID-19 cases was 8.3%, with the highest frequency observed during the 5th wave, coinciding with the arrival of the SARS-CoV-2 omicron variant. Incidence rates of recurrent cases were 6.7 cases per 100 persons per wave, 24% lower than the general population's incidence rate (8.8 cases per 100 persons per wave) (p<0.05). Statistically significant differences in incidence rates between age groups were observed, reaching 14.2 cases per 100 people per year, or 2.6 times higher. Conclusions. When assessing vaccination efficacy in real-life cohort studies, it is advisable to calculate rates separately for each COVID-19 wave. During the fifth and subsequent waves, the risk of COVID-19 development and other outcomes characterizing severity and consequences of infection should be considered. Differences in incidence rates between age groups necessitate stratification of compared groups by age and sex.