TUESDAY, Oct. 28 (HealthDay News) -- A high dose of seasonal flu vaccine gives an added boost to the immune system of people aged 65 and older and provides them with better protection, according to a U.S. study that included nearly 4,000 people.

Blood tests showed that participants who received a dose four times greater than the standard flu vaccine had 30 percent to 80 percent more antibodies against flu than those who received the standard dose. The larger dose contained 60 micrograms of material designed to evoke an immune response to protect a person against a particular type of flu, while the standard dose contained 15 micrograms.

The level of antibodies in the blood has long been considered a good indication of how much protection people have against the flu.

Participants who received the larger dose were more likely to experience the typical side effects associated with a flu shot, such as redness, swelling and pain at the injection site. But the symptoms generally didn't last for more than three days, the researchers said.

The findings were presented Sunday at a national meeting on infectious diseases, in Washington, D.C. The study was funded by Sanofi-Aventis, which makes a widely used flu vaccine.

"Without doubt, the influenza vaccine as it is today is beneficial for everyone, including older adults, and we strongly encourage every older person, and every person with a chronic illness, to get vaccinated," study author Dr. Ann Falsey, an associate professor of medicine at the University of Rochester (N.Y.) School of Medicine and Dentistry, said in a university news release.

"However, older people generally don't respond to vaccines as well as young healthy adults and therefore, there is much room for improvement. The goal is to increase immune response in older adults, since this is one of the populations most at risk for becoming seriously ill or dying from influenza," Falsey said.

In the United States, people 65 and older account for about 90 percent of the estimated 36,000 people who die from flu-related causes each year, according to background information in the news release.

More information

The U.S. National Institute on Aging has more about flu shots.