Incidence of influenza among patients admitted to the Department of Internal Medicine during the winter season

Background: Influenza is a common disease, especially in the winter season. Influenza virus causes symptoms of acute viral illness, development of complications and worsening of chronic diseases. Vaccination may prevent influenza illness. Objectives: 1. To determine the rate of vaccination against influenza. Furthermore, in the immunized patients - the study aimed to determine the efficiency of the vaccination. 2. To determine the incidence of influenza illness among patients admitted to a Department of Internal Medicine during the winter season. Methods: We performed a prospective screening of 124 consecutive patients hospitalized in the Department of Internal Medicine during February 1998. Demographic parameters (age, sex) and admission diagnosis were collected. All patients were asked about influenza vaccination in the last fall. Blood samples were collected on admission and 10-30 days later. Three relevant virus subtypes were examined. Virus subtype was determined in all patients with evidence of influenza illness. Results: Eleven patients (8.9%) were found with evidence of acute influenza illness at the time of hospitalization. The rate of patients with chronic pulmonary disease was higher in this group than in the group without evidence of influenza (p=0.03). Forty-four out of 124 patients (35.5%) had been vaccinated against influenza. In the vaccinated patients, the protective antibody level for all 3 subtypes developed in 27.3% of cases and for one or two subtypes in 50% of cases. In 22.7% of patients no protective level of antibody was detected for any subtype. Conclusions: It is necessary to encourage vaccination in the high-risk group patient, especially those with chronic pulmonary disease. It is essential to develop new vaccination strategies and prophylactic antiviral medications.