Glossary
12 TERMS · PLAIN LANGUAGEThe field's jargon, translated. Each term gets the precise definition and a plain-language line — because an honest record should be readable by the people it's about.
- Vaccine efficacy (VE)VE
- The percentage reduction in infections in the vaccinated group versus the placebo group, measured under trial conditions. RV144's 31.2% means 31.2% fewer infections than placebo — modest, and at the edge of statistical significance.Put simply: how much the vaccine cut the risk, compared to no vaccine, among trial volunteers.
- Phase I / II / IIIPH1 PH2 PH3
- Clinical trial stages: Phase 1 tests safety and immune response in a few dozen people; Phase 2 refines dose and safety in hundreds; Phase 3 (or 2b 'proof-of-concept') tests whether it actually prevents infections in thousands.Put simply: is it safe → what dose → does it work.
- FutilitySTOP RULE
- A statistics-based decision that a trial cannot possibly show benefit even if it runs to completion. 'Stopped for futility' is not about danger — it's about absence of effect. HVTN 702 (2020) is the famous example.Put simply: the data already say this won't work, so we stop early and tell everyone.
- gp120GP120
- The envelope protein HIV uses to latch onto immune cells — the target of the first-generation vaccines (VaxGen). Antibodies against it failed to prevent infection; the protein's shape-shifting is a key reason.Put simply: the doorknob HIV uses to enter cells; early vaccines aimed at it and missed.
- Prime-boostALVAC+
- A two-part vaccination strategy: one shot ('prime') starts the immune response, a different one ('boost') sharpens it. RV144 used ALVAC (a weakened bird virus carrying HIV genes) plus AIDSVAX (gp120 protein).Put simply: lesson one, then the exam — two different shots teaching the same subject.
- Broadly neutralizing antibodies (bnAbs)BNABS
- Rare antibodies that disable many HIV variants — the 'elite' weapons the immune system rarely makes on its own, usually only after years of infection in a fraction of people. Modern vaccine design tries to train any body to make them.Put simply: master-key antibodies that work against many HIV strains — the whole game of current research.
- Germline targetingGT
- The strategy of nudging the right young immune cells ('germline' B cells) through a sequence of antigens so they mature into bnAb producers. IAVI G002 (mRNA-based) showed the first steps working in humans in 2025.Put simply: a training ladder for immune cells — first published results say the bottom rungs work.
- Mosaic vaccineMOSAIC
- A design using computer-assembled 'mosaic' antigens patched from many HIV strains, hoping broader coverage beats HIV's diversity. Tested in Imbokodo and Mosaico; both failed (2021, 2023).Put simply: a composite sketch of the virus, meant to work against all disguises. It didn't.
- PrEPPREP
- Pre-exposure prophylaxis: HIV medication taken by HIV-negative people to prevent infection. Now available as a daily pill (2012), a two-monthly injection (2021), and a twice-yearly injection (2025).Put simply: protection before exposure — the parallel line that delivered.
- VectorVECTOR
- A harmless virus or other carrier used to deliver HIV's genetic material into cells so the immune system learns it. Ad5 (STEP) and Ad26 (Mosaico) are examples; MHRP's ALVAC used a canarypox vector.Put simply: a delivery truck carrying HIV blueprints so your body holds a fire drill.
- Correlate of protectionCORRELATE
- A measurable immune signal (antibody level, cell response) that predicts whether a vaccinated person is protected. HIV has no confirmed correlate — a core reason progress is slow.Put simply: the field still can't check a blood test and say 'you're protected.'
- DSMBDSMB
- Data and Safety Monitoring Board: an independent committee that reviews unblinded trial data and can order a stop. DSMBs stopped STEP (2007) and HVTN 702 (2020).Put simply: independent referees with power to end the game early.
Missing a term? Write to [email protected]. Definitions follow
primary literature; where a term has contested usage, we say so on the
evidence page.