Influenza vaccination is safe and immunogenic when administered to hospitalized patients.

Abstract Context: Very few high-risk persons receive influenza vaccine while hospitalized. Health care providers may be reluctant to administer the influenza vaccine to hospitalized patients because of insufficient data related to the safety and immunogenicity of vaccinating this population. Objective: To evaluate the safety and immunogenicity of the influenza vaccine administered to hospitalized patients compared to ambulatory clinic patients. Design: Prospective cohort study. Setting: A 711-bed, acute-care, private tertiary hospital and two private internal medicine clinics from October 1 to December 25, 1999. Participants: 51 inpatients; 177 outpatients. Intervention: All study participants received influenza vaccine. Serum samples for antibody assays were collected at baseline and at 3 weeks after vaccination. Study participants were mailed a questionnaire to elicit information about adverse effects of vaccination. Main Outcome Measures: Seroconversion rates (4-fold increase in hemagglutination-inhibiting antibodies) and seroprotection rates (titers ≥1:40) to the influenza vaccine in hospitalized and ambulatory patients. Results: The seroconversion and post-vaccination seroprotection rates in the inpatient group were 65% (33/51) and 88% (45/51) compared to 55% (98/177) and 94% (167/177) in the ambulatory clinic patients. Soreness at the site of injection was the most common adverse effect, reported by 12% of both the inpatients (5/42) and ambulatory clinic patients (20/171). Seventy-four percent of patients (31/41 inpatients and 127/174 outpatients) reported no significant side effect to vaccination. Conclusions: Influenza vaccine is at least as safe and immunogenic in hospitalized patients as it is in ambulatory patients. These data can be used to help convince health care providers to order the influenza vaccine for their hospitalized patients.