When flu season strikes, the first line of defense for seniors, who are considered among the most vulnerable, is a flu vaccine.

But a new study by Group Health suggests that for seniors, a vaccine doesn't offer as much protection as originally thought.

The study, which will be published in Saturday's issue of the medical journal The Lancet, found no link between flu vaccinations and the risk of pneumonia - a common and potentially life-threatening complication of the flu.

The study looked at 3,519 seniors aged 65 to 94 years old enrolled in Group Health during the 2000, 2001 and 2002 pre-flu and flu seasons. The study compared two groups of patients: those with pneumonia treated in a hospital or elsewhere and people matched to the first group by sex and age but with no pneumonia. All had intact immune systems, and none lived in nursing homes. Both groups had the same rates of the flu vaccine, but there was no significant difference in the pneumonia rates.

"This suggests that the flu vaccine doesn't protect seniors as much as has been thought," said Dr. Michael Jackson, who was a postdoctoral fellow at the Group Health Center for Health Studies and lead author of the study, which he calls the largest case-control study of the flu vaccine in the elderly.

The Group Health study was developed after other research studies were reporting more-than-realistic benefits of the flu vaccine, so he and his colleagues began looking for biases in those studies, said Jackson, now an epidemiologist for the Centers for Disease Control and Prevention.

Jackson said the biggest problem was the studies only looked at seniors treated for pneumonia in hospitals, excluding patients treated in an outpatient setting. They also included seniors who had immune-system problems and didn't review medical records to get information on chronic conditions, such as heart or lung disease, that raise the risk of pneumonia. Also, the previous studies didn't look at the differences between healthier seniors and those who were frail, which could prevent them from leaving home and getting a flu vaccine.

"If you don't account or identify all of these people, it's easy to overestimate the results," Jackson said. "Hopefully this puts a more realistic stance on this."

Dr. Jeff Duchin, chief of communicable disease control for Public Health - Seattle & King County, said the study adds important information to the understanding of the influenza vaccine in seniors but shouldn't stop anyone from getting a flu vaccine every year.

The Group Health study comes on the heels of other research saying that the evidence indicating that all older people need flu vaccines is weak. In an article published in October's issue of Lancet Infectious Diseases, which included comments from Dr. Lisa Jackson, a senior investigator at the Group Health Center for Health Studies, researchers said that only observational studies, not gold-standard clinical trials, have been used to defend the benefits of older people getting flu vaccines.

They cite past studies, including those conducted by the Centers for Disease Control and Prevention, showing that despite a jump in vaccinations in the elderly from 15 percent to 65 percent since 1980, flu deaths among elderly people increased during the 1980s and 1990s. A similar study in Italy also found no decrease in deaths from flu among the elderly, despite an increase in vaccines by 60 percent in that population. Researchers say those results raise questions about whether the flu vaccine helps prevent death in older people.

Jackson said the bottom line is seniors should still be vaccinated against the flu, but the Group Health study shows researchers have work to do to figure out what else can work to help reduce the risk of infections.

"The next step is to find complementary things that can be done such as hand-washing or continuing to vaccinate people who come into contact with seniors," he said. "We don't have data to say which one way complements best, but from the scientific side, there's been a lot of talk of what the next step should be."