"This suggests that the flu vaccine doesn't protect seniors as much as has been thought. Ours is by far the largest case-control study of flu vaccine in the elderly," The Lancet quoted Michael L. Jackson, PhD, MPH, a postdoctoral fellow at the Group Health Center for Health Studies, as saying.
Jackson and his colleagues carefully reviewed medical records to reveal details of seniors' health and ability to do daily activities.
"We tried to overcome the limits of previous studies done by others. Those studies may have overestimated the benefits of the flu vaccine in the elderly for various reasons," he said.
For instance, those studies looked only at pneumonia cases treated in a hospital.
They also included seniors who had immune problems, which limit potential benefit from vaccination.
And they didn't review medical records to get information on chronic diseases, such as heart or lung disease, which raise the risk of pneumonia.
Jackson said that most importantly, those previous studies also failed to account for differences between healthier seniors and those who were 'frail.'
Frail seniors are older and have chronic diseases and difficulty walking.
"They are less likely than younger, healthier seniors to go out and get vaccinated-and more apt to develop pneumonia," he said.
Jackson said Pneumonia is a common and potentially life-threatening complication of the flu. But pneumonia can happen without the flu.
"That's why our study used a control time period, after flu vaccine became available but before each flu season actually started," he said.
During those pre-flu-season periods, people who had been vaccinated were much less likely to get pneumonia because those who got the vaccine happened to be healthier-not because the flu vaccine was protecting them from pneumonia caused by the flu, since it wasn't present yet, he explained.
"Despite our findings, and even though immune responses are known to decline with age. The flu vaccine is safe. So it seems worth getting, even if it might lower the risk of pneumonia and death only slightly," he said.
The study is published in the August 2 issue of The Lancet.