TUESDAY, Sept. 25 (HealthDay News) -- The ability of flu shots to reduce the risk of influenza-related death among people 65 and older may be greatly exaggerated, say researchers who reviewed the available scientific evidence.
This exaggeration is the result of "frailty selection bias" -- where not-so-frail older people are vaccinated more often than those who are infirm, the team said. It's also due to the use of non-specific clinical trail endpoints, such as death from any and all causes, wrote a group led by Dr. Lone Simonsen, of George Washington University in Washington, D.C.
They reported their conclusions in the October issue ofThe Lancet Infectious Diseasesjournal.
The researchers noted that few clinical trials of flu vaccines have included seniors, especially those aged 70 and older -- the age group that accounts for 75 percent of all influenza-related deaths. Those studies that have been conducted with seniors suggested a steady decline in clinical benefits and vaccine antibody responses after age 70.
The review authors also found that, even though vaccination rates increased from 15 percent to 65 percent since 1980, studies have not been able to confirm a corresponding decline in flu-related deaths.
"Paradoxically, whereas those studies attribute about five percent of all winter deaths to influenza, many cohort studies report a 50 percent reduction in the total risk of death in winter -- a benefit ten times greater than the estimated influenza mortality burden," the review authors wrote.
Future studies on flu vaccines in older groups should use more specific endpoints and actual virus surveillance data for each flu season, rather than the current arbitrary four-month period, the team added.
Despite the current lack of evidence, the review authors do recommend that seniors continue to get flu shots.
"While awaiting an improved evidence base for influenza vaccine mortality benefits in elderly people, we suggest that this group should continue to be vaccinated against influenza. Influenza causes many deaths each year, and even a partly effective vaccine would be better than no vaccine at all," they said. "But the evidence base concerning influenza vaccine benefits in elderly people does need to be strengthened."