NEW YORK -- July 31, 2008 -- The effect of influenza vaccination on the risk of pneumonia in immunocompetent elderly people during influenza seasons might be less efficacious than previously estimated, according to a study in the August 2 edition of The Lancet.
Previous studies have shown that older people who were vaccinated had a 20% to 30% reduced risk of hospital admission for pneumonia compared with older people who had not been vaccinated. However, these findings could have been biased by underlying differences in health status between the groups studied.
Michael Jackson, MD, Group Health Center for Health Studies, Seattle, Washington, and colleagues assessed whether influenza vaccination is associated with a risk of community-acquired pneumonia in immunocompetent elderly people after controlling for health status indicators.
The study involved a group of individuals, aged 65 to 94 years, enrolled in the Group Health Cooperative during the 2000 through 2002 preinfluenza and influenza seasons.
Cases of inpatient or outpatient pneumonia were validated by review of medical records or chest radiograph reports. Two age- and sex-matched controls were randomly selected for each case.
Medical records were also reviewed to define potential confounders, including smoking history, presence and severity of lung and heart disease, and frailty indicators.
The researchers investigated 1,173 cases of community-acquired pneumonia and 2,346 controls. After the results were adjusted for the confounding effects of the presence and severity of other diseases in the patients, as defined by chart review, influenza vaccination was not associated with a reduced risk of community-acquired pneumonia.
"These results could mean that influenza infection only causes a small proportion of the pneumonias in these people, so that reduction of the risk of influenza infection does not lead to an important reduction in all-cause pneumonia," the authors wrote.
Alternatively, these results could mean that the vaccine is not very effective in reducing the risk of influenza infection in older people at risk of pneumonia. These 2 possibilities have quite different implications for vaccine development and vaccination recommendations.
In an accompanying comment, Edward Belongia, MD, Marshfield Clinic Research Foundation, Marshfield, Wisconsin, and David Shay, MD, Influenza Division, US Centers for Disease Control and Prevention, Atlanta, Georgia, said, "More studies that use laboratory-confirmed outcomes and adjust for a broad range of confounding variables will provide valuable information about the effects of antigenic match and other factors that affect vaccine effectiveness in elderly adults."
SOURCE: The Lancet
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