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.