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Older, more infirm seniors may be less likely to receive influenza vaccine
Study biases may account for a ‘healthy vaccine effect,’ according to researchers.
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WASHINGTON — Influenza vaccinations increased until about age 80 in women and age 85 in men and then declined, according to findings were presented at the 2008 ICAAC/IDSA Meeting. The findings also indicated that healthier people may be more likely than their less healthy peers to receive influenza vaccination.
Researchers conducted a retrospective cohort study involving about 347,000 Kaiser Permanente Northern California members aged older than 65 years. They found that influenza vaccination rates increased up to about the 90th percentile for infirmity, then decreased.
The researchers observed a similar curvilinear relationship in mortality risk as they did in influenza vaccination. Vaccination rates began to decrease when probability of death increased to greater than 5%, according to the findings. “This could produce a ‘healthy vaccinee effect’ in observational studies, making the vaccine look more effective than it actually is,” the researchers wrote.
“Our studies show that Jackson et al and Simonsen et al are probably correct in that there is a great deal of selection bias in observational studies,” Roger Baxter, MD, co-director of the Kaiser Permanente Vaccine Study Center, told Infectious Disease News. “This may account for some of what has been perceived as effectiveness.”
Confounding factors
The researchers also observed that even after adjusting for age and infirmity level, vaccine effectiveness at preventing death remained difficult to estimate due to residual confounding factors. One of those confounding factors may involve the difference between vaccine effectiveness rates prior to the influenza season as opposed to rates during the influenza season. As a result, the true effectiveness of the influenza vaccine in the elderly is difficult to determine. These factors appear to be related to people not getting the vaccine near the end of life, which may be due to frailty, mobility, or just giving up.
“We still do not know how well the vaccine actually works in seniors, specifically in preventing death,” Baxter said. “But even a very small benefit in overall death is worth it, and we should continue to vaccinate seniors while looking for better options including adjuvants and the vaccination of schoolchildren. We do know that vaccination appears to decrease in those who are likely to die, so it makes it appear that the vaccine has protected from death.” – by Rob Volansky
For more information:
- Baxter R. Who gets flu vaccines? A look at bias in flu vaccine effectiveness studies. Presented at: the 2008 ICAAC/IDSA Meeting; Oct. 25-28, 2008; Washington, D.C.
- Simonsen L, Taylor R, Viboud C, et al. Mortality benefits of influenza vaccination in elderly people: an ongoing controversy. Lancet Infect Dis. 2007;7:658-666.
- Jackson L, Jackson M, Nelson J, et al. Evidence of bias in estimates of influenza vaccine effectiveness in seniors. Int J Epidemiol. 2006;35:337-344.