As flu sweeps across the country, many health experts are worried that the shots taken by tens of millions of Americans will offer considerably weaker protection than any flu vaccine in recent years.
The flu shot available this year was formulated to protect against three strains of the virus.
But the strain actually circulating this year is somewhat different from those three, and it probably is too late to develop a new formula.
Even though one of the three is a close cousin of this season's bug, whether that will be enough to help people ward off the flu is simply unknown. Some experts expect the level of protection to be 50 percent or less.
Most agree the vaccine will do at least some good. But the word ''some'' is about as specific as scientists at the Centers for Disease Control and Prevention are willing to get.
Even in the best of years, the flu shot is not foolproof. Ordin-arily it is between 70 percent and 90 percent effective in healthy young adults and somewhat less effective in the elderly.
The last time the vaccine missed the mark was in 1997, when a strain called A-Sydney appeared that was significantly different from the strain included in the flu shot. Doctors still are uncertain about how much good the shot did that season, although the CDC estimates it was 30 percent to 50 percent protective.
Scientists say this year's mismatch does not seem to be as great as it was six years ago, but they cannot say with certainty how well the flu shot will work, or even whether it will.
''With a vaccine with a less optimal match, you have to say it might not work at all,'' said Dr. Scott Harper, a CDC epidemiologist.
''That's very unlikely. Prob-ably it will not be 90 percent effective. But we just have no good sense of how it will work in humans. Biology is messy.''
The mismatch occurred when experts decided to include a strain called A-Panama in this year's shot, the same as last year. It turns out that the dominant virus this fall has been A-Fujian, which has two genetic mutations that make it different.
A variety of studies using ferrets, the primary lab animal for flu experiments, and human blood suggest that the A-Panama shots should offer at least partial protection against A-Fujian. The most encouraging predictions come from the World Health Organization, which, along with the U.S. Food and Drug Administration, decides what strains of flu virus will be included in the shot.
Dr. Klaus Stohr said blood taken from people who got this year's shot shows they have 41 percent fewer antibodies against A-Fujian than they do against A-Panama, and three-quarters of vaccinated adults have enough antibodies to protect against A-Fujian.
''The data indicate that the vaccine in use does protect to a good extent against the currently circulating strain,'' Stohr said.
However, Harper said human blood samples and ferret experiments cannot be reliably used to predict how well a vaccine will work.
The CDC is doing several studies to estimate how well people are protected in areas where the flu is circulating, but the answer probably will not be known until the end of the flu season.
Dr. Walt Orenstein, director of the CDC's National Immun-ization Program, said it is probably too late to create an additional vaccine for this season, although manufacturers are working with the A-Fujian strain.
''I won't rule it out, but it's unlikely,'' he said.
Dr. Sandra Kemmerly, associate head of infectious diseases at the Oschner Clinic in New Orleans, said there is already anecdotal evidence that the shots are helping people escape the worst of the flu.
''Of the people we are seeing who were vaccinated and have flu-like symptoms, it is much milder than if they had not been vaccinated at all,'' she said.
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