From respiratory diseases to nervous system disorders: Unraveling the certified causes of influenza-associated deaths in Poland from 2000 to 2019.

Background: This study aims to accurately estimate influenza-associated deaths in Poland and their certified cause of death, due to significant discrepancies between official numbers and expected impact. Methods: Excess influenza-associated mortality in Poland from 2000 to 2019 was calculated using Seasonal-Trend Decomposition Procedure based on LOESS (STL), which can detect non-linear trends and non-sinusoidal cycles. Excess mortality was then used as an explanatory variable in a model predicting monthly fluctuations of officially recorded causes of death from 2010 to 2019. Results: A total of 142,000 conservative estimates of influenza-associated deaths were identified, representing 1.86% of overall mortality. Only 0.61% of influenza-associated deaths were officially recorded as influenza. Nearly half of certified influenza deaths were attributed to the seasonal baseline mortality, potentially doubling estimated impact based solely on influenza peaks. Influenza-associated deaths were frequently recorded as respiratory diseases (24.36%), with majority attributed to underlying conditions such as cardiovascular diseases (45.31%), cancer (9.06%), or diabetes (2.66%). Influenza-associated deaths were more commonly certified as nervous system diseases (1.84%) or mental disorders (1.04%), rather than influenza itself. There was a noticeable impact of influenza on secondary infections, such as meningococcal and gastrointestinal infections. Conclusion: These findings highlight the importance of improved estimation for informing public health policy decisions.