The purpose of this research is to evaluate blood samples from HIV infected and non-HIV infected people opioid and non opioid users to understand how opioid affect the immune responses (body defenses against infection) to the flu vaccine.

Inclusion Criteria:

For Opioid (OP) users/non-users:

1. OP users - prescribed opioids for at least the past 90 days; or injecting opioids for at least 90 days
2. Opioid never-users in the past year

Additional criteria for OP users:

1. OP use for 90 days pre-flu vaccination
2. 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:

1. 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
2. CD4 count available in the prior 6 months and >200/mm3
3. 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:

1. Individuals age 18-60 yrs .
2. No history of other immunodeficiency disorders
3. Not on steroid or other immunosuppressive/immunomodulators medications.
4. No active malignancies.
5. No contraindication to receive influenza vaccination (allergy to chicken eggs or to any other substance of the vaccine).
6. Agreeable to receive the influenza vaccination.
7. Agreeable to participate in study for a complete course of study full visits.
8. Able to provide informed consent.

Exclusion Criteria:

1. Contraindication to receive influenza vaccination (allergy to chicken eggs or to any other substance of the vaccine).
2. Non-adherence to ART for HIV+
3. Unable to provide informed consent.
4. Other comorbid conditions such as diabetes mellitus type 2 (DMT2)
5. Influenza vaccination already given during the current vaccination season.