[Stirner]

The article does not explain how they find out if someone who was vaccinated is (or is not) still immune. They can't expose the person to the virus. Maybe they expose a tissue sample to the virus or expose the person to a similar but non-harmful virus and look for an immune response.

[realpatriot71]

You can expose the person to viral proteins and look for an immune response specifically a quick IgG reponse.