Excess hospitalizations and mortality associated with seasonal influenza in Spain, 2008–2018

Abstract

Background Influenza may trigger complications, particularly in at-risk groups, potentially leading to hospitalization or death. However, due to lack of routine testing, influenza cases are infrequently coded with influenza-specific diagnosis. Statistical models using influenza activity as an explanatory variable can be used to estimate annual hospitalizations and deaths associated with influenza. Our study aimed to estimate the clinical and economic burden of severe influenza in Spain, considering such models. Methods The study comprised ten epidemic seasons (2008/2009–2017/2018) and used two approaches: (i) a direct method of estimating the seasonal influenza hospitalization, based on the number of National Health Service hospitalizations with influenza-specific International Classification of Diseases (ICD) codes (ICD-9: 487–488; ICD-10: J09-J11), as primary or secondary diagnosis; (ii) an indirect method of estimating excess hospitalizations and deaths using broader groups of ICD codes in time-series models, computed for six age groups and four groups of diagnoses: pneumonia or influenza (ICD-9: 480–488, 517.1; ICD-10: J09–J18), respiratory (ICD-9: 460–519; ICD-10: J00–J99), respiratory or cardiovascular (C&R, ICD-9: 390–459, 460–519; ICD-10: I00–I99, J00–J99), and all-cause. Means, excluding the H1N1pdm09 pandemic (2009/2010), are reported in this study. Results The mean number of hospitalizations with a diagnosis of influenza per season was 13,063, corresponding to 28.1 cases per 100,000 people. The mean direct annual cost of these hospitalizations was €45.7 million, of which 65.7% was generated by patients with comorbidities. Mean annual influenza-associated C&R hospitalizations were estimated at 34,894 (min: 16,546; max: 52,861), corresponding to 75.0 cases per 100,000 (95% confidence interval [CI]: 63.3–86.3) for all ages and 335.3 (95% CI: 293.2–377.5) in patients aged ≥ 65 years. We estimate 3.8 influenza-associated excess C&R hospitalizations for each hospitalization coded with an influenza-specific diagnosis in patients aged ≥ 65 years. The mean direct annual cost of the estimated excess C&R hospitalizations was €142.9 million for all ages and €115.9 million for patients aged ≥ 65 years. Mean annual influenza-associated all-cause mortality per 100,000 people was estimated at 27.7 for all ages. Conclusions Results suggest a relevant under-detected burden of influenza mostly in the elderly population, but not neglectable in younger people.

Addressing influenza’s underestimated burden – Iberian experts call to action

“…National Health Service (NHS) hospitalizations coded with influenza-specific diagnosis were compared with estimates on the excess hospitalization and mortality attributable to influenza based on time series ecological models. Results have been published for Spain and Portugal [ 14 , 15 ]. Complementarily, an electronic medical records (EMR) database from four Spanish regions during one season (2017/18) was analysed, shedding light on the direct healthcare burden of medically attended influenza, beyond hospitalization and using longitudinal health care data [ 16 ].…”

“…Influenza was estimated to be responsible for 34,894 respiratory and/or cardiovascular (C&R) hospitalizations on average over 9 influenza seasons in public hospitals per year in Spain and 5,356 in Portugal [ 14 , 15 ]. In the deadliest season (2014/15), 24,268 excess all-cause deaths attributable to influenza were estimated in Spain and 5,016 in Portugal [ 14 , 15 ]. It led to a substantial pressure on NHS resources due to influenza during winter.…”

“…It led to a substantial pressure on NHS resources due to influenza during winter. Patients stayed at the hospital on average 9.4 days in Spain and 10.7 days in Portugal, both above the respective national means for all-causes and ages [ 14 , 15 ].…”

“…Patients with comorbidities accounted for 59.0%/65.6% of NHS hospitalizations coded as influenza and for 65.7%/78.6% of their costs (in Spain/Portugal). 14,15 Although most hospitalizations and deaths were driven by population aged ≥ 65 years old, population from 50 to 64 years with comorbidities was a particularly vulnerable group [ 14 , 15 ]. In both countries, the VCR in this age group is lower than in the ≥ 65 years old group, namely, in season 2017/2018: 55.9% in those ≥ 65 years vs. 22.1% in population aged 60–64 years in Spain [ 17 ]; and 61.2% vs. 31.8% in Portugal [ 18 ], respectively.…”

“…Our findings further support that the burden of influenza goes beyond respiratory complications, [ 3 ] and that its impact is not adequately captured by the current hospitalization and deaths codification practices, nor by analysing only other respiratory diagnoses. Unlike what was observed during the COVID-19 pandemic – where testing became more generalized –data from the BARI study suggests that many hospitalizations and deaths triggered by influenza were still left unidentified [ 14 , 15 ]. Hospitalizations coded as influenza represented only 37.4%/27.3% of the estimated excess C&R hospitalizations attributable to influenza in all-age group and 26.3%/14.0% in population aged ≥ 65 (in Spain/Portugal).…”