The timing of flu is very unpredictable and can vary in different parts of the country and from season to season. Most seasonal flu activity typically occurs between October and May. Flu activity most commonly peaks in the United States between December and February.

Pediatricians can and should still take action to protect children from influenza late in the flu season. Because influenza occurs in many areas of the world during April through September, vaccine should be given to travelers who missed vaccination in the preceding fall and winter. Children younger than age 9 years who needed 2 doses of vaccine but have not received it yet, can still get it now.