• CTL vaccines may not prevent HIV infection completely. But they will hopefully get the immune system to kill HIV-infected cells efficiently enough to turn HIV-infected people into the equivalent of 'long-term non-progressors'. They might also work as a therapeutic vaccine, slowing down disease in someone already infected.
• However, HIV can sometimes evade recognition by the CTL cells. It mutates so that cells infected with it stop having the 'signature' CTL cells recognise.
• Also, there are many different sub-species - 'clades' - of HIV, and controversy is at present raging as to whether vaccines designed to combat clades common in the developed world will work against African and Asian strains of HIV, and vice versa.

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Prime-boost vaccines

• The vaccine strategy thought most likely to work is what is called a 'prime-boost' strategy. 'Naked' HIV components will be injected, followed within months by boosters of the same components wrapped inside a harmless virus. This could generate both kinds of immune response.
• The world's second phase III trial, which will start this year in Thailand, is of a prime-boost vaccine. This ALVAC vaccine will prime its subjects with a bit-of-HIV-inside-canarypox-virus vaccine, twice, and then follow up three and six months later with boosts of the VaxGen gp120 vaccine. Four injections in all.
• The next most advanced HIV vaccines in the pipeline - one produced by Merck and one overseen by IAVI on trial in the UK and Kenya, have already produced CTL responses in 60-80 per cent of trial subjects. But no one knows if those responses are sufficient to prevent a disease-causing HIV infection.