###HUMAN: what is the number of patients whose ethnicity is hispanic/latino - puerto rican and drug name is influenza virus vaccine?, ###ASSISTANT: SELECT COUNT(DISTINCT demographic.subject_id) FROM demographic INNER JOIN prescriptions ON demographic.hadm_id = prescriptions.hadm_id WHERE demographic.ethnicity = "HISPANIC/LATINO - PUERTO RICAN" AND prescriptions.drug = "Influenza Virus Vaccine"