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 and 50,000 people in the United States each 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 on 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. Against the influenza A (H3N2) strain, the vaccine protected only 47 percent.
The protective benefits of the vaccine 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, a finding that was statistically insignificant.
The estimates are based on studies of 2,697 children and adults enrolled in the U.S. Influenza Vaccine Effectiveness Network 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 unprotected during this year's flu season.
POOR IMMUNE RESPONSE TO VACCINES
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 in the report 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."
Dr. Joseph Bresee, a flu expert at the CDC, said overall the vaccine worked "OK," but it is not fully clear why people over 65 responded poorly.
"Part of it is they are elderly and they respond less vigorously to vaccines. But it appears that some of the people, at least, developed a less robust response to this particular antigen," he said, referring to the H3N2 component of the flu vaccine.
"Why that is, I don't think we know yet. We're looking at it closely," he said.
Bresee said most of the elderly in the study took conventional flu vaccine, and not the high-dose version developed by vaccine giant Sanofi to address issues of poor immune responses in the elderly.
"We'd love to be able to look at the question of whether it is actually more effective or not. We just don't have enough use yet," he said.
Sanofi said it shipped 6 million doses of the special formulation for the elderly, but it did not have information on how many of the doses were used, or whether any had been used by patients in the study.
What is clear, Bresee said, is the need for the elderly who get sick with flu symptoms to seek treatment with antiviral medications, such as Roche Holding Ag's Tamiflu, which can reduce the severity of their illness.
CDC noted that vaccine 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 - known as a quadrivalent vaccine - instead of three strains, known as a trivalent vaccine. 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 quadrivalent vaccines will add an additional B strain.
Last month, the FDA approved the first gene-based flu vaccine developed 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 an interview last month.
Glaxo's Fluarix 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 Sanofi is also in the process of having their four-strain 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 its high dose vaccine for 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."
(Additional reporting by Bill Berkrot in New York; Editing by Mohammad Zargham)