TUESDAY, Oct. 25 (HealthDay News) -- The most widely used flu vaccine in the United States is only about 60 percent effective in healthy adults, new research indicates.

That vaccine is the trivalent inactivated vaccine (TIV), which accounts for about 90 percent of flu shots in this country, according to a report in the Oct. 25 issue of The Lancet Infectious Diseases.

The less common live attenuated influenza vaccine (LAIV) is 83 percent protective in children aged 7 and younger, but it's not recommended for everyone in this age group.

The findings are in line with other recent reviews and don't mean that people shouldn't get their annual flu shot, said Dr. Joseph Bresee, chief of the epidemiology and prevention branch in the influenza division of the U.S. Centers for Disease Control and Prevention.

"The findings are not that unexpected, but flu vaccines do work. They don't work as well as we'd like them to work all the time, [but] flu is a bad disease. It can cause death and hospitalization, and the flu vaccine is absolutely the best tool to prevent that," Bresee said. "While we all want them to be better, they're still the best thing we have."

Since 2010, U.S. health authorities have recommended that all individuals aged 6 months or older receive a flu vaccine, either the traditional shot (this is the inactivated vaccine) or the nasal-spray flu vaccine, which is made with live albeit weakened influenza viruses. The nasal spray is only approved for people aged 2 through 49.

The inactivated vaccine has been in use since 1978, and represents the lion's share of all influenza vaccines in the United States. Only about 9 percent of vaccines are given in the live LAIV form, which was approved for use in 2003.

One of the reasons the live vaccine is not used as much, despite its greater effectiveness, is that it's only licensed for those aged 2 to 49 and for those without underlying high-risk health conditions, Bresee explained, which limits its use somewhat.

"I am sure that there may be other reasons for relatively little use, even among those eligible, but it is a good vaccine, and induces good protection for those eligible to receive it," he said.

The new meta-analysis included data from 31 previous studies on the flu vaccine conducted over the past 40 years. The flu shot was shown to be 59 percent effective in adults aged 18 to 65.

There were no trials in children 2 to 17 years old or adults aged 65 and over.

The vaccine for the pandemic H1N1 flu was about 69 percent effective, which the study authors said was "not adequate for a pandemic setting."

"It's a classic glass-half-full, glass-half-empty situation," said Dr. John J. Treanor, a professor of medicine and of microbiology and immunology at the University of Rochester Medical Center in New York. "Almost every study they looked at showed some level of efficacy. Flu vaccines definitely work to prevent influenza. The argument is about just how effective are they."

But the study does "highlight the need for better flu vaccines," said Bresee.

According to Treanor, there are several different vaccine approaches scientists are researching.

One is just using a higher dose and, in fact, there is a high-dose product on the market right now for seniors. The vaccine does produce higher levels of antibodies, but it's not clear yet if this translates into better protection against disease, Treanor said.

Another strategy is to make more potent adjuvants, which are compounds added to vaccines to enhance their effectiveness.

More information

The U.S. Centers for Disease Control and Prevention has more on the annual flu vaccine.