Acute respiratory illness among immunized and nonimmunized patients with high-risk factors during a split season of influenza A and B.

A prospective survey of acute respiratory illness (RI) was made among 329 vaccinated and unvaccinated ambulatory patients with high-risk factors for influenza. Surveillance for virus during the influenza season revealed the predominance of influenza A and B in sequential periods. During the influenza A period, febrile RI was greatest among patients with chronic pulmonary disease; among 66 patients, vaccination significantly reduced RI. Age greater than 65 y (199 patients) and heart (90 patients) and metabolic diseases (151 patients) did not increase the relative frequency of febrile RI, and vaccine administration caused no apparent reduction in frequency of RI. During the influenza B period, no differences in RI were observed between the groups, and the frequency of RI was unrelated to vaccination. Variability in virus and vaccine specificities may have been important. Except for one subset of specific virus and host conditions, no overall reduction in influenza-like or total RI was observed from vaccinating ambulatory, high-risk patients.