From the Indian ministry of health:

Meaures shall be taken to strengthen the immune system through a healthy diet and lifestyle practises. Agastya Harityaki 5 gm, twice a day with warm water.

Samshamani Vati 500 mg twice a day.

Trikatu(Pippali, Marich & Shunthi) powder 5 gm and Tulasi 3-5 leaves (boiled in 1-litre water, until it reduces to ½liter and keeps it in a bottle) keep taking it in sips as and when required.

Pratimarsa Nasya: Instill two drops of Anu taila/Sesame oil in each nostril daily in the morning.