NEW YORK (Reuters Health) – Influenza vaccination may not be as effective in reducing pneumonia incidence among immunocompetent elderly individuals as previously reported, according to research published in the August 2 issue of The Lancet.

The authors note that in observational studies, influenza vaccinations were associated with fewer hospital admissions for pneumonia. Those studies, however, failed to control for functional impairments and illness severity and did not consider pneumonia cases not requiring hospitalization.

In their paper, Dr. Michael L. Jackson at Group Health Center for Health Studies in Seattle and colleagues describe a population-based, nested case-control study conducted in western Washington state during the 2000, 2001, and 2002 preinfluenza periods and influenza seasons. They note that the vaccines in those years were well matched to the circulating virus strains.

The cohort comprised community-dwelling, immunocompetent patients aged 65 to 94 years, including 1173 individuals with community-acquired pneumonia that occurred between the date the influenza vaccine first became available and the end of that year’s influenza season, and 2346 age- and sex-matched controls.

After controlling for comorbidities, frailty, medication use, and smoking, vaccination was not associated with a significant reduction in the risk of pneumonia. The same held true for analyses restricted to the peak influenza seasons and for pneumonia cases requiring hospital admission.

The investigators propose two possibilities to explain their findings: Either influenza causes a small proportion of pneumonia incidents, or else the vaccine is not very effective in reducing risk of influenza infection in elderly people at risk of pneumonia.

In a Lancet commentary, Dr. Edward A. Belongia, at the Marshfield Clinic Research Foundation in Marshfield, Wisconsin, and Dr. David K. Shay at the US Centers for Disease Control and Prevention in Atlanta discuss the need for further research into causes of pneumonia and other influenza-related acute illnesses among elderly adults in highly vaccinated populations.

According to the editorialists, “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.”

Lancet 2008;372:352-354,398-405.