There's little evidence that flu shots protect small kids, says a respected medical review group.

Despite the finding, the CDC and the American Academy of Pediatrics see no reason to change their strong 2004 recommendation. This makes flu vaccinations for all children aged 6-23 months a top priority. As a result, 57 percent of American kids under the age of 2 got flu shots for the 2004-2005 flu season.

The report comes from researchers associated with the Cochrane Collaboration (search). This independent, international medical group strongly advocates that all medical treatments should be based on strict evidence of safety and effectiveness.

After reviewing every study they could find on flu vaccination of healthy children, Tom Jefferson, MD, and colleagues find that flu shots work quite well in children older than 2 years. But it's a different story for younger children.

"We found no evidence supporting vaccination of children younger than two years," Jefferson tells WebMD. "That does not mean the vaccine does not work. It means the evidence is not there. The vaccine may very well prevent flu transmission, death, and hospitalization — but we could find no evidence of this."

Jefferson and colleagues' preliminary report — their full review will come later this year — appears in the Feb. 26 issue of The Lancet.

Challenges Faced by Researchers

One reason why the Cochrane reviewers failed to find evidence supporting flu shots for small children was that the data simply isn't there. And the reports that do exist, Jefferson says, used such different methods that it is difficult to compare one with the other.

Flu vaccines work when they prevent flu infection or make the flu less severe in those who do catch the bug. Relatively few vaccine studies actually test people who get sick to see whether their illness is, in fact, the flu. That means that many studies simply look at the number of people who get flu-like illness.

"Think about a year's respiratory infections as a pie," Jefferson says. "A third are caused by unknown agents. Another third are the various common cold agents. And the remaining third is a mixed bag of viruses including flu viruses – the amount varies each year. The symptoms for all the infection in this pie are all the same. They vary slightly, of course, but you can't tell these bugs apart by the symptoms they cause."

Studies that look at whether a flu vaccine reduces the number of flu-like illnesses may give a good idea of whether the vaccine works in adults. But kids are different, says Keith R. Powell, MD. Powell, chairman of pediatrics at Northeastern Ohio University College of Medicine, sat on the American Academy of Pediatrics committee that made the flu shot recommendations.

"The average kid has eight respiratory infections a year," Powell tells WebMD. "It would be very difficult to say there are going to be fewer respiratory infections in kids who get flu vaccine. … You could have a bad year for respiratory syncytial virus and say this, hey, this stuff didn't work at all."

Flu Shot Safety Not an Issue

Carolyn B. Bridges, MD, a medical epidemiologist in the CDC's influenza branch, says the Cochrane team was too quick to discount some very good evidence that flu shots do indeed work in small kids. Bridges points to a recent study showing that flu vaccination is 66 percent effective in preventing flu infection in young children.

Jefferson says his team finds that flu shots work better in older children.

"Beyond 2 years of age, as the child gets older, the vaccine gets more efficacious," he says.

Branch says flu is simply too serious a disease to fool around with. That, she says, played a big role in the 2004 recommendations by the U.S. Advisory Committee on Immunization Practices (ACIP).

"The ACIP voted to recommend vaccination of children aged 6-23 months because of the burden of severe flu disease in children of that age," she says. "Children younger than 2 years have very high flu hospitalization rates. We felt there was sufficient data on safety — it is safe in that age group — and that we have sufficient data on effectiveness, and that we had a high burden of disease. So that is why ACIP moved forward."

Jefferson agrees with Branch that there is no reason to doubt the safety of flu shots for kids, even those aged 6-23 months.

"We found no evidence of major adverse events from the studies we reviewed," he says. "We have no reason to believe these vaccinations cause anything major."

Jefferson is quick to note that the Cochrane reviewers are not calling for a change in the current recommendations. What they do want is more research.

"I think the bottom line is we don't know enough about this," he says. "The question now is, what you do in a situation of uncertainty? No one has the answer."

By Daniel J. DeNoon, reviewed by Brunilda Nazario, MD

SOURCES: Jefferson, T. The Lancet, Feb. 26, 2005; vol 365: pp 773-780. CDC, Influenza Vaccination among Adults and Children during the 2004-05 Influenza Season, Feb. 10, 2005. American Academy of Pediatrics, Pediatrics, May 2004; vol 113; pp 1441-1447. Hoberman, A. Journal of the American Medical Association, Sept. 24, 2003; vol 290: pp 1608-16. Tom Jefferson, MD, Cochrane Vaccines Field, Rome. Carolyn B. Bridges, MD, medical epidemiologist, influenza branch, NCID, CDC. Keith R. Powell, MD, chairman of pediatrics, Northeastern Ohio University College of Medicine; and member, American Academy of Pediatrics Committee on Infectious Diseases.