(CBS/AP) A new government report on the effectiveness of this year's flu vaccine finds dramatic discrepancies in the amount of protection Americans received, with senior citizens being left the most vulnerable.
The 2012-2013 influenza vaccine contained two influenza A strains and one influenza B strain. The A strains included an H1N1 (swine flu) strain similar to the one that caused a 2009 pandemic and a new H3N2 strain, that officials later discovered to be behind much of the serious illness reported this year. The vaccine also contained a 2010 influenza B strain.
The new report from the Centers for Disease Control and Prevention found the vaccine offered 58 percent protection against the most common and dangerous H3N2 strain for children ages 6 months to 17 years old, 46 percent protection for adults ages 18 to 49, and 50 percent protection for adults 60 to 64 years of age.
However, for seniors 65 and older, this year's flu shot was found to be only 9 percent effective against the more virulent H3N2 strain, the report showed.
Overall, the flu vaccine was found to be 56 percent effective at reducing the need for medical visits caused by the illness. That's around the initial 62 percent effectiveness figure the CDC reported in January based on early test results collected from 1,155 children and adults who went to doctors with respiratory infections.
In recent years, the vaccine has been about 60 to 70 percent effective at preventing flu.
For adults 65 and older, the vaccine was found to be 27 percent effective against the three strains, according to the new report -- the lowest in about a decade, but not far below from what's expected.
But the vaccine did a particularly poor job of protecting older people against the harshest flu strain, and CDC officials say it's not clear why.
The findings were published Feb. 21 in the CDC's journal, Morbidity and Mortality Weekly Report.
"There's obviously bad news and there's some better news that we have to remind ourselves of," Dr. William Schaffner, an infectious disease researcher at Vanderbilt University who served as past president of the National Foundation for Infectious Diseases, told CBSNews.com Thursday.
Schaffner serves on the CDC's Advisory Committee on Immunization Practices, which selects the strains included in the vaccine each year. At a committee meeting today where the new data was presented, Schaffner described the mood as unhappy, with experts "scratching their heads" as to why the vaccine's effectiveness was "unexpectedly low" against the virulent H3N2 strain for this age group.
"All of these experts in the room were looking at each other stunned," he said.
According to Schaffner, studies have showed the flu vaccine to be about 40 percent effective overall in the elderly. He expressed optimism to CBSNews.com in January because the H3N2 influenza A strain the committee selected prior to the start of the flu season was a "bull's-eye hit" for the strain causing most of the illnesses this year. That remains true, he said Thursday, which is why today's findings in seniors were so puzzling.
As dismal as the numbers looked, he said, there needs to be perspective. The vaccine was still 27 percent effective overall for seniors over 65 years. For that age group, there aren't other preventive disease-fighting approaches as effective, he said. He also added that the better protection rates reported in children and adults under 65 suggest people in those age groups were less likely to give influenza to older people.
"You have to add in all the benefits before you give the influenza vaccine a thumbs down," said Schaffner. Play Video
Flu death rate among seniors highest ever.Flu surveillance reports released by the CDC in recent months have shown adults 65 and older account for more than half of the Americans who have been hospitalized because of the flu.
Schaffner explained to CBS This Morning earlier this month that seniors have been hit hardest because their immune systems are more frail than those of younger more robust people, and this year's flu strain was more virulent than past years, meaning it was more likely to cause serious illness.
"That's a nasty combination," he said at the time. The new report's findings that the vaccine wasn't very effective against H3N2 likely contributed to seniors being hit hardest this flu season, he said Thursday.
Throughout the flu season, the CDC recommended adults over 65 contact their doctor at the first sign of illness to be put on an antiviral medication such as Tamiflu or Relenza, because the antivirals may reduce the severity of disease and prevent complications like pneumonia.
The flu vaccine: What's stockpiled, why you should get one now
Among infectious diseases, flu is considered one of the nation's leading killers. On average, about 24,000 Americans die each flu season, according to the CDC.
This flu season started in early December -- a month earlier than usual -- and peaked by the end of the year. The flu remains an epidemic, accounting 9.1 percent of all U.S. deaths during the week of February, 3 to 9, according to recent CDC estimates. However, the number of states showing widespread activity has fallen over recent weeks.
It's important to note the new study of seniors is less than definitive -- only 290 adults over 65 were included in the analysis.
"It's important to remember that this 'effectiveness rate' refers to prevention of outpatient medical visits due to the flu. We do not yet have this year's statistics on how effective the vaccine was in preventing serious complications such as pneumonia and death," CBS News medical correspondent Dr. Jon LaPook told CBSNews.com. "Based on previous flu seasons, it is likely that even when patients get the flu despite receiving the flu vaccine, the vaccine still cuts down on the severity of the disease."
A flu vaccine is considered pretty good for seniors if it's in the 30 to 40 percent range, Dr. Arnold Monto, a University of Michigan flu expert told the Associated Press.
Older people have weaker immune systems that don't respond as well to flu shots. That's why a high-dose version was recently made available for those 65 and older. The new study was too small to show whether that made a difference this year.
The CDC estimates are based on about 2,700 people who got sick in December and January. The researchers traced back to see who had gotten flu shots and who hadn't.
"Year in and year out, the vaccine is the best protection we have," CDC flu expert Dr. Joseph Bresee, told the AP.
Bresee said there's a danger in providing preliminary results because it may result in people doubting -- or skipping -- flu shots. The data was released to warn older people who got shots that they may still get sick and shouldn't ignore any serious flu-like symptoms, he said.
The CDC told Dr. LaPook that better vaccines are needed and that this is the subject of very active research, adding it was "disappointed" by the low effectiveness rate against this year's most common flu strain. But, there is definite benefit to the vaccine "and it continues to be the most important weapon," according to the agency.
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