They found that annual flu shots appear to reduce the risk of developing lower respiratory tract infections among healthy individuals age 65 and older during years with mild to moderate influenza epidemics, but this benefit was not evident among subjects with other illnesses.

Despite this finding of modest risk reduction, reported in the Archives of Internal Medicine, Dr. Bettie C. G. Voordouw and colleagues still recommend flu vaccinations for all elderly subjects because the researchers had previously established that these treatments reduced mortality rates.

The team evaluated the extent to which annual influenza vaccination alters the overall risk of developing an infection among 26,071 community-dwelling individuals age 65 and older, who had medical records in the database at the researchers' institution, Erasmus University Medical Center in Rotterdam during the period from 1996 to 2002. Approximately half of the subjects had other illnesses.

During the study, more than 59,000 flu vaccinations were administered to almost 21,000 patients. Out of the entire cohort, some 3400 developed a first episode of pneumonia, acute bronchitis or exacerbation of chronic bronchitis.

Overall, influenza vaccination did not alter the risk for developing one of these lung conditions.

However, the team noted a 33 percent reduction in risk of lung infections in moderate flu seasons following any flu vaccination or revaccination among the subjects who were otherwise healthy. On the other hand, subjects with pre-existing illness were not protected in the same way.

Nevertheless, Voordouw's group still recommends that, "although the protective effect is modest, influenza vaccination should be advised in view of the high background incidence of lower respiratory tract infection and because of the observed reduced mortality in elderly individuals."