Hi !!
There are various homeopathic medicines for Flu prevention
Aconite, Arsenic, Gelsemium
For Homeopathic
Calcium Supplement you can give Calc Phos 6x 2 tablets morning and evening
Ayurvedic home remedies for VIT C like Amla or Indian Gooseberries, Lemon, Guava, Oranges are excellent source
Health Tips
In 1 glass of Luke warm water add 1 tablespoon honey and 2 spoon of lemon juice for ur daily Vit C dose