Why do children under 9 receiving the influenza vaccine for the first time need two doses?

Question: 
Why do children under 9 receiving the influenza vaccine for the first time need two doses? 
 
Answer: 

Children under 9 need 2 doses of the influenza vaccine the first year they receive it for best protection. The first dose of the influenza vaccine “primes” the immune system but may not result in high levels of protection.  So, a second dose is needed to raise their level of protection. The second dose results in the child's immune system producing enough antibodies so they will be able to fight off influenza when they come in contact with it.

Date last reviewed: 
Wednesday, Nov 03, 2021