The effect of the COVID-19 pandemic on influenza-related hospitalization and in-hospital fatality in Poland: A nationwide register-based study

Background: Despite vaccination being widely recognized as the most effective measure to prevent influenza and its complications, many countries still have suboptimal vaccination rates. On review of the data on influenza vaccine coverage in Poland during the pandemic era (2020/2021, 6%; 2021/2022, 7%), influenza vaccine coverage in Poland was higher compared to the pre-pandemic period (2019/2020, 4%). We determined the impact of the COVID-19 pandemic on influenza-related hospitalization and in-hospital fatality in Poland by comparing hospital admissions and in-hospital death rates during the pandemic era (04.03.2020-31.12.2022) to the preceding four years (01.01.2016-03.03.2020).

Methods: Data on influenza-related hospitalizations and in-hospital deaths among patients in Poland were obtained from the Nationwide General Hospital Morbidity Study held by the National Institute of Public Health, and the study period included seven influenza seasons (2016 -2022).

Results: The total number of influenza-related hospitalizations in the study period was 39,604; 36.6% were pandemic-era hospitalizations. Influenza hospitalization rates ranged from 7.9 to 17.1 per 100,000 in 2016-2019. In 2020, the hospitalization rate increased to 21.1 per 100,000. In the following year, we observed a significant decrease to 1.3 and an increase to 30.3 per 100,000 in 2022. During the pandemic, 297 influenza-related in-hospital deaths were reported vs. 849 in the pre-pandemic period. The annual in-hospital fatality rates in the study group decreased from 3.1% -4.5% in 2016-2019 to 1.5% -2.9% in 2020-2022.

Conclusions: We report a significant decrease in influenza-related hospitalizations and in-hospital deaths in 2021. This decrease is likely related to an increased vaccine uptake and non-pharmacological public health interventions (e.g., facemasks, distancing, hand-washing, and symptomatic isolation) implemented during the COVID-19 pandemic. Reduced global influenza incidence worldwide and in Poland after the outbreak of the COVID-19 pandemic increased in 2022 after the easing of COVID-19 restrictions.