Hey dear as a mother I understand ur suitation abht kids health. Dear healthy diet s very necessary for improves immunity .so for improves immunity give healthy diet as add milk ,cheese butter,paneer,green vegetables,fresh fruits,nuts, oats ,ragi,sooji all r very necessary for improving immunity..try it dear nd yes influenza vaccine s very necessary for improves immunity too .so consult to ur pediatrician nd go 4 influenza vaccine too .I also gives these diet nd influenza vaccine too u can try it dear..