The vaccine produced an immune response in children 10 to 17 years old in just 10 days, but younger children had weaker responses. They will likely need two shots 21 days apart, officials said.
The findings are something of a surprise to infectious disease experts who initially assumed that children of all ages would likely need two doses of the vaccine, since their immune systems are less mature than adults.
But so far, the immune response to the H1N1 inoculation is "acting strikingly similar" to seasonal flu shots, said Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases. Typically, younger children who have never received a seasonal flu shot before must have two shots to spark an adequate immune response.
The findings mean that younger children could need four shots this fall, to protect against the seasonal flu and the swine flu.
The preliminary results come from data gleaned at NIH-sponsored trials of a vaccine made by French manufacturer Sanofi Pasteur. The trials are being conducted at the University of Maryland's Center for Vaccine Development and a handful of other sites. The studies tested two doses of the vaccine (15 micrograms and 30 micrograms) on 600 children of three age groups: 10 to 17 years old, 3 to 9 years old, and 6 months to 36 months.
In children 10 to 17 years old, 76 percent had a strong immune response to one 15-microgram dose. But for kids 3 to 9 years old, only 36 percent had a strong immune response with the same dose. In the youngest group, a single 15-microgram dose of vaccine produced a strong immune response in just 25 percent of recipients.
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