Epidemiology of Hospitalizations Related to Coding (ICD-9-CM) of the Influenza Diagnosis in Infants up to 6 Months of Age in Spain (2009 - 2015)

Background: Influenza is a vaccine-preventable respiratory infection that results in substantial morbidity and mortality each year. Although infants below 6 months of age have the highest hospitalization rates for their increased risk of developing severe influenza, the existing influenza vaccines are not licensed for this group of age.

Objectives: To analyze hospitalization episodes with an ICD-9 diagnosis code of influenza (codes 487 and 488) in any diagnostic position in infants up to 6 months of age from 2009 to 2015 in the Spanish hospital surveillance system.

Methods: Retrospective survey by reviewing data of the National Surveillance System for Hospital Data including more than 98% of Spanish hospitals and 99.5% of the population in Spain. It contains data about admission and discharge date, age, sex, diagnosis and discharge status for all hospitalizations in our country. All hospitalizations related to coding (ICD-9-CM; 487, 488) of the influenza diagnosis in infants up to 6 months of age were analyzed.

Results: A total of 2,739 hospital admissions were obtained and analyzed. Of those, 2,676 admissions corresponded to diagnoses ICD-9 code 487 (97.7%) and 63 (2.3%) corresponded to ICD-9 code 488. Males were 59.6% and females were 40.4%. The global median age was 1.7 months and the average inpatient length of stay was 6.8 days (95% CI, 6.5 – 7.1). A total of ten deaths were recorded among the inpatients throughout the entire period, corresponding to an inpatient hospital fatality rate of 0.4%.

Conclusions: In our study, ICD-9-CM codes 487 and 488, related to influenza diagnosis, are present in a high number of hospitalizations in infant below 6 months of age. Improving preventive strategies such as increasing vaccination coverages in elderly population and cocoon immunization strategies may reduce these hospitalizations.