The following demographic variables and pre-existing medical conditions were recorded for all study participants: age, sex, ethnicity, educational level, smoking, alcoholism, pregnancy in women aged 15 -49 years, history of pneumonia in the previous two years, chronic obstructive pulmonary disease (COPD), asthma, cardiovascular disease, renal failure, diabetes, HIV infection, disabling neurological disease, cancer, transplantation, morbid obesity (body mass index ≥40), use of neuraminidase inhibitors before hospital admission (and their timing relative to the onset of symptoms, verified after contacting the prescribing general practitioner), use of other medications in the 90 days before hospital admission (corticosteroids, antibiotics etc.) and treatment received during hospitalization (medications, catheters and mechanical ventilation). For each vaccine, a case was considered vaccinated if the vaccine had been received ≥14 days before the onset of symptoms. Data were collected during hospital admission and the clinical chart was also reviewed after discharge.