⁃ For Opioid (OP) users/non-users:

• OP users - prescribed opioids for at least the past 90 days; or injecting opioids for at least 90 days

• Opioid never-users in the past year

⁃ Additional criteria for OP users:

• OP use for 90 days pre-flu vaccination

• Continued OP use for 4 weeks post flu vaccination

⁃ For HIV positive participants:

⁃ 1\) HIV infection, as documented by any licensed ELISA kit and confirmed by Western blot at any time prior to study entry. Participants on ART as a result of prior HIV documented infection will not be required to provide proof of diagnosis of HIV infection.

⁃ Additional criteria for HIV positive participants:

• On ART for at least 1 year with plasma pending viral load (VL) \<200 copies/mL. Occasional viral blips up to 1000 copies/ml also acceptable provided the patients are on continuing treatment

• CD4 count available in the prior 6 months and \>200/mm3

• Undetectable viral load (\< 200 copies/mL)

⁃ For HIV negative participants:

⁃ 1\) Documented negative HIV test, either by any licensed ELISA or rapid tests within the past 6 months.

⁃ For all participants:

• Individuals age 18-60 yrs .

• No history of other immunodeficiency disorders

• Not on steroid or other immunosuppressive/immunomodulators medications.

• No active malignancies.

• No contraindication to receive influenza vaccination (allergy to chicken eggs or to any other substance of the vaccine).

• Agreeable to receive the influenza vaccination.

• Agreeable to participate in study for a complete course of study full visits.

• Able to provide informed consent.