Studies of influenza vaccine effectiveness in elderly people substantially overestimate vaccine benefits, according to new research from the US published in the International Journal of Epidemiology ( IJE ).
Research by Lisa Jackson and colleagues at Group Health Co-operative, Seattle, found evidence of serious bias in estimates of influenza vaccine effectiveness in seniors.
Their study suggests that the association between vaccination and risk of death is influenced by the fact that relatively healthy seniors ( that is, those already less at risk from dying ) are more likely to get vaccinated.
However, the research does not indicate that there is no benefit in vaccinating the elderly, just that the differences in health between seniors who receive vaccine and those who do not make it difficult or impossible to tell what benefit is being derived from the vaccine.
A commentary on the papers argues that there are reasons to seek ways to augment current vaccination strategy, for example, vaccinating health care workers, nursing home personnel and school children, who are the major spreaders of flu within the community. This would further decrease flu deaths in the elderly by reducing transmission of the virus. It could be done while better vaccines for the elderly are developed along with the use of new technologies to better evaluate effectiveness.
Jackson and colleagues studied 73,527 people aged 65 and over during an eight year period. They evaluated the association of flu vaccination and risk of death, and the association of flu vaccination and risk of hospitalisation for pneumonia, in periods before, during and after flu season.
Since a protective effect of vaccination should be specific to flu season, they would expect to find reductions in risk of death or hospitalisation during the season. However, they found the greatest reductions in the period before flu season when there should be no true vaccine effect.
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, 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 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 strategy is that high-risk persons are relatively inaccessible; no improvement in vaccine coverage has occurred since 1997. School children could be accessible through school-based vaccine clinics allowing rapid administration of vaccine to large numbers, representing all socioeconomic groups, within a short period of time."
Source: University of Bristol, 2005