From the CDC website - 2 doses may be necessary for kids of a certain age.

Will two doses of vaccine be required?

The U.S. Food and Drug Administration (FDA) has approved the use of one dose of 2009 H1N1 flu vaccine for persons 10 years of age and older. This is slightly different from CDC’s recommendations for seasonal influenza vaccination which states that children younger than 9 who are being vaccinated against influenza for the first time need to receive two doses. Infants younger than 6 months of age are too young to get the 2009 H1N1 and seasonal flu vaccines.
What will be the recommended interval between the first and second dose for children 9 years of age and under?

CDC recommends that the two doses of 2009 H1N1 vaccine be separated by 4 weeks. However, if the second dose is separated from the first dose by at least 21 days, the second dose can be considered valid.
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