Analysis of influenza epidemics during the COVID-19 pandemic using an improved surveillance system (from 2021 to 2024)

Aim. Assessing the effectiveness of new baselines and intensity thresholds of epidemics based on rates of incidence and hospitalization with a diagnosis of “influenza” to clarify the timing of epidemics and their spread throughout the Russian Federation against the backdrop of the COVID-19 pandemic (from 2021 to 2024). Materials and methods. At the A.A. Smorodintsev Influenza Research Institute, the software was reformed taking into account the need to solve the tasks set during the COVID-19 pandemic. Due to changes in influenza surveillance in relation to the diagnosis of “influenza”, and hence the increase in the registration of influenza cases, the baseline and threshold of epidemics were adjusted for the influenza incidence and hospitalization rates in the surveyed cities in total (61) and for each Federal districts (over the entire population and by age groups) for 3 epidemics against the background of COVID-19 pandemic (2021–2022, 2022–2023, and 2023–2024). Results. A comparison of baselines calculated over the previous 6 seasons based on the incidence and hospitalization rates of mostly clinically diagnosed influenza and new baseline levels of incidence and hospitalization rates of mostly laboratory-confirmed influenza during the pandemic showed minor changes in the indicators of baseline incidence and hospitalization rates while epidemic thresholds for these indicators increased. Conclusion. Against the backdrop of the COVID-19 pandemic during the 2020–2021 season, there was no influenza epidemic. In 2021–2022, the A(H3N2) epidemic was of low intensity in terms of incidence, hospitalization rates and low mortality (2 cases). In 2022–2023, the influenza A(H1N1)pdm09 and B epidemic was of moderate intensity in terms of incidence, with a high frequency of hospitalizations and mortality (120 cases). Influenza A(H3N2) and B epidemic in 2023–2024 was of a “very high” level in terms of the influenza incidence, but average in terms of the frequency of hospitalizations and mortality (41 cases), with higher incidence rate compared to the previous epidemic (0.28% and 0.19% of the total population), including persons over 15 years of age (0.19% and 0.12%).