Influenza immunization is recommended for elderly individuals and residents of long-term care facilities, but its effectiveness remains controversial, and randomized trials are scarce. In a systematic review, investigators analyzed 49 nonrandomized cohort studies that compared influenza vaccination to no intervention in more than 2 million people aged 65 or older.
In 25 studies conducted in long-term care facilities during flu outbreaks, recipients of vaccines that were well matched to circulating strains had 23% less influenza-like illness, 46% less pneumonia, 45% fewer hospitalizations for influenza and pneumonia, and 42% fewer deaths from influenza and pneumonia. Seven studies conducted in long-term care facilities during periods of low viral circulation suggested significant vaccine effectiveness for various endpoints, but these studies were small or potentially biased.
In 20 studies of elderly patients living in the community, inactivated vaccine was ineffective against influenza, influenza-like illness, pneumonia, and death from respiratory disease. Recipients of well-matched vaccines had 26% fewer hospitalizations for influenza and pneumonia. However, after adjustment for confounders in 7 studies, vaccine recipients had 47% fewer all-cause deaths, and fewer were hospitalized for influenza and pneumonia (27% reduction), respiratory disease (22%), and cardiac disease (24%).
The cohort studies analyzed here, particularly those in community settings, are subject to selection bias and other biases, which probably led to overestimation of treatment effect. Nevertheless, the data suggest that immunization is useful in long-term care facilities and modestly effective elsewhere.
Reference:
Jefferson T et al. Efficacy and effectiveness of influenza vaccines in elderly people: A systematic review. Lancet 2005 Oct 1; 366:1165-74.