• Amounts of drugs used
  
• Response of viral load to retreatment after interruption
  

• Opportunistic infections and deaths
  
• Adverse effects
  
• CD4 counts
  
• Resistance development
  

Continuous treatment has been very successful in diminishing the diseases and deaths caused by HIV. However, continuous treatment is expensive. Intermittent treatment will always cost less than continuous treatment, and therefore has the potential to facilitate access to highly active antiretroviral therapy (HAART) in developing countries. HAART also causes many undesirable effects. Intermittent treatment decreases exposure to drugs and is therefore expected to decrease side effects.

STACCATO is a randomised trial of intermittent versus continuous anti-retroviral treatment. At least 600 patients on HAART, with viremia below 50 copies/ml and CD4 count above 350 cells/ml were randomised to one of two arms, in 1:2 proportions:

• Arm 1: Continuation (control) arm: Drugs are continued or changed according to current guidelines and good clinical practice.
• Arm 2: CD4-guided arm: Drugs discontinued and reintroduced according to CD4 counts, with HAART being administered only if CD4 count is < 350 cells/ml.

Randomized treatment will continue during an average of approximately 2 years, and will be followed by a period of 12 to 24 weeks’ continuous treatment, for patients in both arms.

Endpoints: The amount of drugs used, side effects, viremia and CD4 counts, number of clinical events, at the end of the randomized treatment period, and again 12 to 24 weeks later. A subproject will study the effect of treatment interruption on resistance development, mutations in proviral DNA and proviral DNA levels.