Similarly, there is a little evidence that vaccinating healthcare workers protects their elderly patients from flu complications, another study found.
The reviews appear in the current issue of The Cochrane Library, a publication of The Cochrane Collaboration, an international organization that evaluates medical research. Systematic reviews draw evidence-based conclusions about medical practice after considering both the content and quality of existing medical trials on a topic.
In their review of 71 studies, Dr. Daniela Rivetti of the public health department of Asti, Italy ,and colleagues found that flu shots prevented 45 percent of flu-like illnesses, hospital admissions and flu-related deaths among nursing home and long-term care patients. Flu vaccinations prevented only 25 percent of these outcomes in older adults still living in the community.
The findings would seem to fly in the face of recommendations by the Centers for Disease Control and Prevention and others who have made regular flu vaccinations among older adults a centerpiece of good public health.
The current recommendations are based in part on tallies of deaths from all causes, in which people who have been vaccinated are among those least likely to die in a given year. However, those who are most likely to get vaccinated are also those most likely to have higher incomes and better health care.
This may mean that the apparent benefits of vaccination could just be a reflection of better health among those who get the shots, Rivetti and colleagues suggest.
Research that focuses on vaccination's effects on all causes of death rather than just flu illnesses "are likely to lead to unrealistic estimates of the effects of the vaccines," Rivetti and colleagues say.
However, studies that compare the effects of flu vaccinations against placebo "are no longer possible on ethical grounds," because of current flu vaccine recommendations, they add.
The review does suggest vaccinating the elderly in institutions such as nursing homes may be effective, preventing about 45 percent of pneumonia cases, hospital admissions and flu-related deaths, the Cochrane reviewers say.
W. Paul Glezen, an infectious disease researcher at Baylor College of Medicine, says the Cochrane review and other recent flu studies "clearly show that better vaccines are needed to protect elderly patients who are particularly vulnerable to complications of influenza."
The CDC and others also recommend flu shots for health workers who care for older adults, although less than 40 percent of these workers were vaccinated in 2003.
A second Cochrane review by Dr. Roger Thomas of the University of Calgary in Alberta, Canada, and colleagues found some evidence that vaccinating the staff of long-term nursing homes can reduce pneumonia deaths, but not proven cases of flu or lower respiratory tract infections among the elderly.
On the whole, there is "no credible evidence" that vaccinating health workers affects flu complications in their elderly patients, Thomas and colleagues conclude.
"There is evidence that vaccinating the elderly works and vaccinating those under 60 works, but not yet really good proof that vaccinating health care workers provides a synergistic benefit for the elderly in institutions," Thomas said.
Greater efforts to vaccinate another high-risk group — schoolchildren — could help protect vulnerable older people, according to Glezen "Many studies have shown that school children have the highest rates of infection with influenza each year and that they are the major spreaders of influenza in the community and introducers into the household," he said.