The lack of effectiveness in children vaccinated during the 2003-2004 and 2004-2005 flu seasons may have been the result of a poor match between the influenza strains in the vaccines and the strains that ended up causing illness in those years, researchers conclude.
At first glance, the data did suggest that children between the ages of 6 months and 5 years derived some protection from vaccination in these years.
But after adjusting for potentially relevant variables, the researchers concluded that "significant influenza vaccine effectiveness could not be demonstrated for any season, age, or setting" examined.
Infectious disease and vaccine expert William Schaffner, MD, of Vanderbilt University Medical Center, tells WebMD that he was very surprised by the finding.
Schaffner did not take part in the study, which appears in the October issue of the Archives of Pediatric and Adolescent Medicine.
"Even when there is a mismatch [between the virus in the vaccine and the virus that makes people sick] we generally see partial protection, and this study doesn't show that," he says.
Kids and Flu Shots
Starting in 2006, U.S. health officials began recommending annual flu shots for all children between the ages of 6 months and 5 years. Before this, the vaccine had been routinely recommended only up until age 2.
Initial vaccination should include two flu shots, given at least one month apart. After this, children, like adults, need only a single flu shot each year.
The viruses in the flu vaccine change every year, based on international surveillance and expert estimations about which types and strains will circulate in a given year.
In an effort to assess the effectiveness of the flu vaccine in young children, Peter G. Szilagyi, MD, and colleagues of the University of Rochester School of Medicine and Dentistry compared 414 children with confirmed cases of the flu seen in hospital emergency rooms or doctors' offices between 2003 and 2005 to a comparison group of more than 5,000 children randomly selected and matched for area of residence.
The children lived in Rochester, N.Y., Nashville, Tenn., or Cincinnati.
After controlling for a number of factors, including county of residence, gender, insurance status, vaccination timing, and whether the children had chronic health conditions that made them more likely to get the flu, the researchers could not show that the vaccine was effective in preventing flu-related medical visits.
They point out that during the 2003-2004 flu season, 99% of circulating influenza strains in the three communities included in the study were due to influenza A virus, but only 11% of the influenza A specimens across the U.S. were similar to a strain included in that year's vaccine.
The next flu season was milder, and Szilagyi and colleague point out that the vaccine was a better match. But, even so, only 36% of circulating virus was similar to vaccine strains.
"The lack of demonstrable vaccine effectiveness in our study may have been due to the sub-optimal match," the researchers write.
But they add that there is also evidence that adults vaccinated during the 2004-2005 flu season did derive some protection.
"It is possible that the vaccine has less effectiveness among children," they write.
Vaccination Still Important
Schaffner says the study was well designed and rigorously carried out. But he worries that the message some will take from the finding is that vaccinating children against the flu is a waste of time.
"Some parents and even some pediatricians are going to ask if vaccinating young children is really worthwhile," he says. "My answer is: 'Absolutely yes.' It is important to point out that more times than not over the last 20 years we have had a very close match between the vaccine and circulating strains."
He points to research suggesting that only about 30% of eligible children get flu shots, and only about half to two-thirds of those vaccinated for the first time get the necessary booster shot.
"Adults have more exposure to real influenza and past vaccines than children, so it makes sense that children might not be as protected," he says.
For the first time this year, government health officials are recommending flu shots for all children over 6 months of age up until their 19th birthdays.
Schaffner says it looks like the upcoming flu season may be a particularly nasty one because new strains of influenza have been circulating in the Southern Hemisphere, where the flu season is ending.
"For the first time in a long time, all three virus strains in the vaccine are new," he says. "My crystal ball suggests that if these new strains are circulating, we may have a more serious influenza season that we have had in many years."