(Reuters) - A U.S. government analysis of this season's flu vaccine suggests it was effective in only 56 percent of people who got the shot, and it completely failed to protect the elderly against an especially deadly strain circulating during flu season.

The U.S. Centers for Disease Control and Prevention said the findings underscore the need for more effective weapons in the fight against influenza, which kills between 3,000 to 50,000 people a year, depending on the severity of the flu season.

"We simply need a better vaccine against influenza, one that works better and lasts longer," CDC Director Dr. Thomas Frieden said in a statement Thursday.

Experts generally estimate the effectiveness of flu vaccines to be between 50 percent and 70 percent, but this vaccine appears to have fallen on the low side of that range.

The vaccine did cut the risk of medical visits caused by either influenza A or influenza B by 56 percent, according to the study published in the CDC's Morbidity and Mortality Weekly Report.

It was more effective against influenza B, protecting 67 percent of those who were vaccinated, compared to the influenza A (H3N2) strain, which only protected 47 percent of those.

The protective benefits of against influenza B were consistent across age groups. That was not the case with the influenza A (H3N2) component of the vaccine, which protected 46 percent to 58 percent of people aged 6 months to 64 years, but only 9 percent of those 65 and older.

The estimates are based on studies of 2,697 children and adults enrolled in the U.S. Influenza Vaccine Effectiveness Network during between December 3 and January 19. The CDC said those estimates may change by the end of the flu season, when more people have been sampled.

Even so, the findings suggest that a large group of elderly people, who are consistently the most vulnerable to influenza, were completely unprotected during this year's flu season.

One possible explanation may be that in older individuals, the immune system often produces a less robust immune response to vaccines, or to any infection.

CDC experts suggested that poor immune response to the influenza A (H3N2) component of the vaccine may help explain why the elderly were not protected, but said the findings "should not discourage future vaccination by persons aged 65 years (or older), who are at greater risk for more severe cases and complications from influenza."

They stressed that flu vaccines remain the best preventive tool available, noting that effectiveness has been known to vary based on a number of factors including virus type, age, the particular flu season and variations in an individual's immunity.

"Although it's far from perfect, flu vaccination is by far the best tool we have to protect from flu," Frieden said.


Frieden said the U.S. Department of Health and Human Services as well as pharmaceutical companies are working to produce better vaccines. Efforts include the use of genetic engineering to develop more potent and more modern flu vaccines, with the hope of ultimately developing a universal flu vaccine that could protect against all strains of flu. Experts predict that could be possible within eight to 10 years.

"It's going to be hard but it's well worth the effort," Frieden said.

Already there are signs of change. In November, the Swiss drugmaker Novartis won U.S. Food and Drug Administration approval for Flucelvax, a seasonal flu vaccine grown in animal cell cultures instead of live chicken eggs, a speedier and more reliable process that could help build stockpiles in the event of a pandemic.

In December, GlaxoSmithKline won FDA approval for a new seasonal flu shot called Fluarix that protects against four strains of seasonal flu instead of three. The announcement followed the approval last February of AstraZeneca's four-strain flu nasal spray made by the company's MedImmune unit. Current vaccines tackled two A strains and one B strain. The quadravalent vaccines will add an additional B strain.

Last month, the FDA approved the first gene-based flu vaccine by privately held Protein Sciences Corp, which uses genetic engineering to grow portions of the virus in insect cells.

"What we're looking at is really incremental improvements, because if we could make the breakthrough improvements easily it would have happened already," Dr. Leonard Friedland, vice president of clinical and medical affairs for vaccines in North America at GlaxoSmithKline, said in a recent interview.

Glaxo's quadravalent vaccine is the first inactivated flu shot to include four instead of three strains of flu.

"It was just licensed and will be available for next season," Friedland said, noting that flu vaccine giant Sanofi is also in the process of having their inactivated quadravalent flu vaccine approved.

AstraZeneca's FluMist, a live, attenuated or weakened flu vaccine, will also have a four-strain version available for next flu season.

Dr. Chris Ambrose, a vice president at AstraZeneca's MedImmune unit, said the company plans to completely switch to the four-strain version of Flumist.

Sanofi Spokeswoman Donna Cary said the company has produced flu vaccines for specific age groups, including a high dose vaccine designed to produce a greater immune response in the elderly.

"The next step for the future is to get to the point where we don't need to develop a new vaccine every year," Cary said. "The main thing we are all looking forward to is the universal vaccine."

(Reporting by Julie Steenhuysen; additional reporting by Bill Berkrot in New York)