You can choose a formula.milk that is rich in prebiotics as it helps in better immunity of the baby. Prebiotics are special plant fibers that help healthy bacteria grow in your gut. This makes your digestive system work better. Prebiotics are a source of food for your gut’s healthy bacteria.

You can choose Aptamil as it's closest to breastmilk and contains prebiotics which strengthens baby's gut and helps boost immunity. But do consult with your doctor before introducing anything new to your baby.