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HIPAA only applies to 'covered entities'. It is also not illegal to ask a person's status.
"If an entity does not meet the definition of a covered entity or business associate, it does not have to comply with the HIPAA Rules."
https://www.hhs.gov/hipaa/for-professionals/covered-entities/index.html
"The Privacy Rule2 does not prohibit any person (e.g., an individual or an entity such as a business), including HIPAA covered entities and business associates, from asking whether an individual has received a particular vaccine, including COVID-19 vaccines."
https://www.hhs.gov/hipaa/for-professionals/privacy/guidance/hipaa-covid-19-vaccination-workplace/index.html