There are no immunity booster medications/rather any medications in the world which can prevent getting a Viral Infection, be it Covid or Influenza.
For prevention --
1) Use Surgical mask whenever you step out
2) Maintain Hygiene(hand washing /sanitization are the best practice)
3) Avoid touching mouth and nose area without washing hand
4) Also ask your family member to follow the same including kids(as kids are generally exposed more)
Now the Vitamins /multivitamins available around are good for your overall health, it make one strong enough to handle our self when we are unwell.
Hence highly recommend to take some multivitamins along with
Vitamin D supplements every yearly if not taken in a while.
To understand the dose of the same, always consult your doctor and take the medications.
Next Steps
Visit /Consult a general physician