Impact of occasional short interruptions of HAART on the progression of HIV infection: results from a cohort study Patrick Taffe Â, Martin Rickenbach, Bernard Hirschel a , Milos Opravil b , Hansjakob Furrer c , Pascal Janin, Florence Bugnon, Bruno Ledergerber b , Thomas Wagels d , Philippe Sudre, and the Swiss HIV Cohort Study  Objectives and design: To investigate the clinical consequences of occasional and short (< 3 months) treatment interruptions in patients having initiated highly active antiretroviral therapy (HAART). Data from the prospective Swiss HIV Cohort Study were used. Methods: Four different endpoints [death, Centers for Disease Control and Prevention (CDC) stages B and C, and CD4 cell count increase > 50 3 10 6 /l] were studied in relation to the number of interruptions that occurred. In order to focus on short interruptions exclusively, observations of patients with a treatment interruption of . 3 months were censored. The CD4 cell count and viraemia were treated as time- dependent variables because of the importance of these factors when an interruption occurs. Results: Between 1 January 1996 and 31 October 2000, 4720 Swiss HIV Cohort Study participants initiated HAART, which was interrupted at least once by 1299 partici- pants. The main reasons for the interruptions were social factors. Interruptions did not increase signi®cantly the risk of HIV-associated morbidity and mortality, except for a marginally increased risk for a CDC stage C event after the ®rst interruption. The ®rst interruption decreased signi®cantly the likelihood of increasing the CD4 cell count. Subsequent interruptions had no further signi®cant effect. High CD4 cell count and low viraemia, assessed as baseline and as longitudinal variables, were associated with a decreased risk of clinical progression. Conclusions: Occasional treatment interruptions of , 3 months neither worsen nor improve disease outcome on an average term (3±4 years). Our results suggest that interruptions might be non-risky, particularly when viraemia is low and CD4 cell count is high. These results require con®rmation. & 2002 Lippincott Williams & Wilkins AIDS 2002, 16:747±755 Keywords: Treatment interruption, HIV, antiretroviral therapy, disease progression Introduction Multiple clinical trials and observational studies have demonstrated the ef®cacy of highly active antiretroviral therapy (HAART) in decreasing HIV-related morbid- ity and mortality [1±3]. However, the potential of such complex therapy is impaired by poor adherence and intolerance leading to treatment interruptions [4±7]. From the Coordination and Data Center, Swiss HIV Cohort Study, Lausanne University Hospital, Lausanne, the a Division of Infectious Diseases, Geneva University Hospital, Geneva, the b Division of Infectious Diseases and Hospital Epidemiology, Zurich University Hospital, Zurich, the c Division of Infectious Diseases, Inselspital, Bern, Switzerland, and d Canton Hospital, St. Gallen, Switzerland.  See Appendix. Requests for reprints to: P. Taffe Â, Coordination and Data Center, Swiss HIV Cohort Study, CHUV, 1011 Lausanne, Switzerland. Received:12 January 2001; revised: 10 August 2001; accepted: 19 September 2001. (see also pp. 787±789) ISSN 0269-9370 & 2002 Lippincott Williams & Wilkins 747