AbstractObjectiveTo estimate mortality attributable to influenza adjusted for othercommon respiratory pathogens, baseline seasonal trends and extremetemperatures.IntroductionAssigning causes of deaths to seasonal infectious diseases is difficultin part due to laboratory testing prior to death being uncommon. Sinceinfluenza (and other common respiratory pathogens) are thereforenotoriously underreported as a (contributing) cause of death in death-cause statistics modeling studies are commonly used to estimate theimpact of influenza on mortality.MethodsUsing primary cause of death (Statistics Netherlands) we modeledweekly timeseries of1) respiratory deaths (ICD10 codes J00-J99) and2) circulatory deaths (ICD10 codes I00-I99).We used regression models with an identity link and Poissonerror to relate mortality to counts of influenza A & B diagnoses.We adjusted for other common respiratory pathogens (all pathogendata was at population level from the national laboratory surveillance),temperature (from the Dutch Royal Meteorological Institute), andbaseline linear and cyclical (i.e. seasonal) trends. To account forthe yearly variation in the severity of the main circulating influenzaA strain we used time dependent variables for influenza A (fixedat lag 0 – assuming a direct effect of influenza. For influenza Band the confoundig pathogens we considered a 0 tot -4 time lag(thus allowing infection to precede death for up to 4 weeks).We performed the analyses separately per death cause group and by3 different age groups (0-64, 65-74,75+ years) over a 14-year time-period (mid 1999-mid 2013, thus 14 complete winter seasons).ResultsIn the Netherlands on average 2,636 all cause deaths occurper week varying by season (lower in summer min: 2,219 and higherin winter max: 3,564) with yearly incidence ranging from 20/10,000in 0-64 year olds to 885/10,000 in 75-plus year olds.Circulatory mortality (31% of total deaths) was higher thanrespiratory mortality (10% of total deaths) and both showed clearseasonality in all age-groups. Overall, 0.14% of all deaths wereactually coded as influenza deaths.Preliminary model estimates showed that the proportion ofrespiratory deaths attributable to influenza A were quite similar for 0-64and 65-74 year olds but higher in 75+ (5.1%, 5.7%, 7.0% respectively)while this proportion was stable across age-groups for circulatorydeaths (approximately 1.5% in all agegroups for influenza A).Influenza B was significantly associated with respiratory deathsand circulatory deaths in the oldest age group of 75+ years(with proportions of 0.7% and 0.2% respectively) while in the65-74 year olds it was associated only with circulatory deaths (0.2%).Influenza B was not significantly associated with either respiratory orcirculatory mortality in the 0-64 year age group.On average, yearly in the 75+ age group 70/10,000 respiratorydeaths and 39/10,000 circulatory deaths were attributable to influenzaA. For influenza B the incidences were 7 to 10 fold lower (7/10,000and 6/10,000 respectively).ConclusionsInfluenza A was significantly associated with respiratory andcirculatory mortality in all age groups while influenza B wassignificantly associated with respiratory and circulatory mortality inthe elderly only.
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