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There is salt rinse available from pharmacies  ( for sinus problems,  not covid) .
You can make your own salt nose rinse. There are instructions on the net, including from our cheng hu (but not in connection with covid).
I haven't tried making it. I only use a nasal spray ( but not for covid) cos I seem to be allergic to the commercially available salt rinse (it is not 100% salt and water)

There are off the shelves for both. salt rinse can be make at home tough. 
All will help together with b cells if ever infected or even vaccinated before.