The reductions in risk before flu season suggest the presence of bias due to preferential receipt of vaccine by relatively healthy seniors on the estimates of flu vaccine effectiveness observed during flu season.
The researchers then looked at 252 people aged over 65 who had died during a flu season and 572 age-matched controls. Using the subjects' medical records, they identified people with 'functional limitations', such as requiring assistance to walk or bathe.
They found that functional limitations were associated with both an increased risk of death and a decreased likelihood of influenza vaccination, and so may confound the association of influenza vaccination and risk of death.
In a commentary on the studies, Professor Paul Glezen of Baylor College of Medicine, Houston said: "During the period from 1989 to 1997 the vaccination rate for people aged over 65 in the US increased from 30 to 67 per cent. Despite this increase, mortality and hospitalisation rates continued to increase rather than decline as would be expected if the vaccine was optimally effective.
"The study by Dr Jackson and her colleagues concludes that elderly people who choose to take the vaccine are, contrary to the findings of earlier cohort studies, in better health than those who fail to get the vaccine. This suggests that better studies of the benefits of flu vaccine in elderly and other high-risk groups are necessary to guide strategies for flu control.
"In the meantime, other strategies should be considered in addition to vaccinating the elderly. Many studies have shown that school children have the highest rates of flu infection each year and that they are the major spreaders of flu in the community and introducers into the household. Immunization of school children, therefore, would reduce exposure of vulnerable patients to flu.
"One of the problems with current vaccination strateg
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Contact: Hannah Johnson
hannah.johnson@bristol.ac.uk
44-117-928-8896
University of Bristol
21-Dec-2005