The swine flu vaccine works in just one dose for older children, but kids younger than 10 will likely need two shots, according to early results of clinical trials, federal health officials said Monday.
The findings mean that younger children could need as many as four shots this fall, to protect against both the seasonal flu and the swine flu.
In the clinical trials, the swine flu vaccine produced a strong immune response in children 10 to 17 years old in just 10 days; younger children generally had a weaker reaction over the same time period. They will likely need two shots given 21 days apart, officials said.
The results in children come from data gleaned at National Institutes of Health-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 healthy children in three age groups: 10 to 17 years old, 3 to 9 years old, and 6 months to 36 months.
In the oldest group, 76 percent had a strong immune response to one 15-microgram dose. But for kids 3 to 9 years old, that figure was 36 percent. Of the youngest volunteers, just 25 percent produced a strong response.
The findings are something of a surprise to infectious disease experts who initially assumed that children of all ages would need two doses of the vaccine, since their immune systems are less mature than those of adults. In fact, up until data two weeks ago showed one dose was strong enough for adults, many experts assumed everyone would need two doses. The H1N1 virus is so different from typically circulating flu strains that most of the population has no immunity to it.
But so far, the immune response to the H1N1 inoculation is "acting strikingly similar" to seasonal flu shots in both adults and children, said Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases.
"Overall, this is very good news for the vaccination program, both for supply of the vaccine and potential efficacy," he said.
Typically, children 9 and under who have never received a seasonal flu shot before must receive two shots to trigger enough immunity to fight off the virus.
