The study findings are to be published in the December issue of the American Journal of Roentgenology.
Also Wednesday, the vaccine maker Sanofi Pasteur said tests are showing that children under 10 years of age may need two doses of the H1N1 swine flu vaccine to be fully protected, the Associated Press reported.
U.S. health officials said the finding isn't surprising because children in this age group typically need two doses of regular seasonal flu vaccine because their immune systems aren't fully developed.
Sanofi is the only company licensed in the United States to make vaccine for children as young as 6 months old. The company tested two strengths of the H1N1 vaccine, given as two shots 21 days apart. The vaccine was tested in 474 children ages 6 months through 9 years old. With one shot, only half of children 6 months to 3 years old had enough immunity, as did three-fourths of children 3 to 9 years old, Sanofi said, the AP reported.
Clinical trials involving adults have shown that one shot is sufficient.
On Tuesday, a leading disease-surveillance expert from Europe reported that the swine flu is killing fewer people than seasonal flu, but is causing greater alarm because it's targeting an unusually large number of children, who typically are less susceptible to flu than older adults.
Denis Coulombier, head of the European Centre for Disease Prevention and Control's preparedness and response unit, told the Agence France-Presse news service that the H1N1 swine flu has killed a very small proportion of those infected with the virus -- about 0.2 to 0.3 deaths per 1,000. That compares to a f
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