Epidemiological factors related to hospitalization due to influenza in children below 6 months of age

The aim of this study was to address the epidemiological factors associated to hospital admissions due to influenza in infants younger than 6 months. A case-control study was performed in a tertiary hospital in Spain. Cases were infants under 6 months of age without comorbidities who were admitted due to influenza between October 2010 and March 2015. Controls were healthy infants younger than 6 months who were hospitalized due to non-respiratory illness or non-infectious diseases (urinary tract infection was included as controls). Data were retrospectively collected from medical records and phone interviews. A total of 88 cases and 122 controls we included. From univariate analysis, differences were found in relation to maternal age (43.1 ± 4.95 vs 32 ± 5.3), paternal age (37 ± 6.4 vs 34.5 ± 6.1), having siblings (79 vs 24%), siblings below 4 years old (54 vs 15%), and having vaccinated grandparents (18 vs 39%) (p < 0.05). After logistic regression, having vaccinated grandparents was an independent protective factor (OR 0.22 [CI95%; 0.05–0.91]), while having siblings was a risk factor (OR 15.8 [CI95% 3.15–79.5]). Vaccination during pregnancy was highly uncommon (3.5 vs 8.3%; p = 0.3).

Conclusion: This study underlines the importance of increasing influenza immunization among household contacts of infants below 6 months to prevent their influenza admission.