From Reuters.com, November 2, 2009 by Thompson Reuters Corporation:

Of the estimated 5 million Americans hit by H1N1, young adults and children are most affected.  Recent government clinical studies have shown that kids of 9 years and under need two doses of the vaccine as more vaccines are distributed.

For more information and statistics about H1H1, see full article: http://www.reuters.com/article/companyNews/idUSN0246253020091102