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Detection of HIV drug resistance during antiretroviral treatment and clinical progression in a large European cohort study

Cozzi-Lepri, Alessandroa; Phillips, Andrew Na; Clotet, Bonaventurab; Mocroft, Amandaa; Ruiz, Lidiab; Kirk, Olec; Lazzarin, Adrianoe; Wiercinska-Drapalo, Alicjaf; Karlsson, Andersg; Lundgren, Jens Dc,d; for the EuroSIDA Study Group

doi: 10.1097/QAD.0b013e328310e04f
Epidemiology and social

Objective(s): To investigate the relationship between detection of HIV drug resistance by 2 years from starting antiretroviral therapy and the subsequent risk of progression to AIDS and death.

Design: Virological failure was defined as experiencing two consecutive viral loads of more than 400 copies/ml in the time window between 0.5 and 2 years from starting antiretroviral therapy (baseline). Patients were grouped according to evidence of virological failure and whether there was detection of the International AIDS Society resistance mutations to one, two or three drug classes in the time window.

Methods: Standard survival analysis using Kaplan–Meier curves and Cox proportional hazards regression model with time-fixed covariates defined at baseline was employed.

Results: We studied 8229 patients in EuroSIDA who started antiretroviral therapy and who had at least 2 years of clinical follow-up. We observed 829 AIDS events and 571 deaths during 38 814 person-years of follow-up resulting in an overall incidence of new AIDS and death of 3.6 per 100 person-years of follow-up [95% confidence interval (CI):3.4–3.8]. By 96 months from baseline, the proportion of patients with a new AIDS diagnosis or death was 20.3% (95% CI:17.7–22.9) in patients with no evidence of virological failure and 53% (39.3–66.7) in those with virological failure and mutations to three drug classes (P = 0.0001). An almost two-fold difference in risk was confirmed in the multivariable analysis (adjusted relative hazard = 1.8, 95% CI:1.2–2.7, P = 0.005).

Conclusion: Although this study shows an association between the detection of resistance at failure and risk of clinical progression, further research is needed to clarify whether resistance reflects poor adherence or directly increases the risk of clinical events via exhaustion of drug options.

aResearch Department of Infection & Population Health, Royal Free and University College Medical School, London, UK

bIrsiCaixa Foundation & Lluita contra la SIDA Foundation, Badalona, Spain

cCopenhagen HIV Programme, Faculty of Health Sciences, Panum Institute, University of Copenhagen, Denmark

dCentre for Viral Diseases@KMA, Rgishospitalet, Copenhagen, Denmark

eInstituto di Ricovero e Cura a Carattere Scientifico, San Raffaele Vita-Salute University, Milan, Italy

fDepartment of Infectious Diseases, Medical University of Bialystok, Bialystok, Poland

gKarolinska University Hospital, Stockholm, Sweden.

*Members of the EuroSIDA Study Group are reported in the Acknowledgments.

Received 21 April, 2008

Revised 11 July, 2008

Accepted 16 July, 2008

Correspondence to Alessandro Cozzi-Lepri, BSc, MSc, PhD, Lecturer in Epidemiology and Biostatistics, Research Department of Infection & Population Health, Royal Free and University College Medical School, UCL, Hampstead Campus/site, Rowland Hill Street, London NW3 2PF, UK. Tel: +44 20 7794 0500 ext 36763; fax: +44 20 7794 1224; e-mail:

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HIV drug resistance tends to accumulate with continuous use of combination antiretroviral therapy (cART) in patients with less than perfect adherence [1–4]. Although the risk of HIV morbidity and mortality is very low in patients treated with cART, a minority of patients still experience disease progression and death [5,6]. One of the possible reasons for long-term failure of cART is the emergence of HIV drug resistance while using cART [7–15]. The rate of HIV drug resistance in clinical practice varies according to specific drugs or class of drugs being used and has been documented in epidemiological studies [16–19]. The use of drug resistance testing is common in clinical practice and recommended by the most recent guidelines for HIV treatment for guiding therapy after evidence of virological failure [20,21]. However, whether the emergence of drug-resistance mutations on cART is associated with an increased risk of clinical progression is still partially controversial [12–15]; in particular, one analysis has shown better clinical outcome in patients in whom resistance was detected compared with those carrying a virus with no resistance mutations [12] and two other analyses identified the detection of non-nucleoside reverse transcriptase inhibitor (NNRTI)-resistance as the strongest predictor of the risk of clinical disease [14,15]. In addition, there is evidence that the mortality risk attributable to drug resistance is low and that the majority of deaths occur in patients who do not take antiretrovirals or take them intermittently [22]. A number of other cohort studies [7–11] have found a strong association between the detection of a multidrug-resistant virus (i.e. a virus susceptible to a limited number of drug classes) and the risk of AIDS and death compared with patients in whom no resistance was detected. The reasons for these observed discrepancies remain unclear but may include the design of the clinical cohort study in question, the statistical approach used or simply a different choice for the comparator group.

Here we studied a well characterized population of HIV-1-infected individuals enrolled in a large European cohort study. The main objective of this analysis was to evaluate whether the detection of HIV drug resistance in patients who have been exposed to cART can help to identify patients who are more likely to subsequently progress to AIDS and death.

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EuroSIDA is a prospective, European study [23] currently including 14 310 patients with HIV-1 infection in 93 centres across Europe (including Israel and Argentina as non-European representatives). Details of the study have been published elsewhere. This analysis includes data up to August 2007 and includes details on all CD4 lymphocyte counts and viral load measured since last data collection, the date of starting and stopping each antiretroviral drug and all AIDS-defining illnesses, using the clinical criteria from 2003. In addition to the clinical database, frozen plasma samples are routinely stored in the EuroSIDA central repository and a proportion of these samples have been sequenced in a central laboratory (IrsiCaixa Foundation, Badalona, Spain). Furthermore, virology reports of genotypic tests performed at local sites are also collected as paper forms and incorporated in the electronic database. In Badalona, HIV-1 RNA is determined using the QIAamp kit (Qiagen Ltd, Madrid, Spain) according to manufacturer's instructions and sequence analysis of HIV-1 encoding protease and reverse transcriptase reading frames is performed using the Trugene HIV-1 Genotyping Kit and OpenGene DNA Sequencing System (version 11.0) according to the manufacturer's recommendations (Siemens Medical Solutions Diagnostics, Barcelona, Spain).

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Baseline for our analysis was 2 years after starting cART (defined as any treatment including ≥3 antiretrovirals).

Virological failure was defined as experiencing two consecutive viral loads more than 400 copies/ml in the time window ranging between 0.5 years after starting cART and baseline (Fig. 1). Patients were categorized into three separate groups according to whether they, between the date of starting cART and baseline, either achieved a viral load of 400 copies/ml or less and showed no evidence of virological failure (group A), showed evidence of virological failure but did not have a genotypic test done in the time window ranging from 0.5–2 years of cART (group B) or showed evidence of virological failure and had at least one genotypic test done in the time window 0.5–2 years (group C; Fig. 1). Only genotypic resistance results performed on samples collected in the time window (0.5;2 years) of cART initiation were used in this analysis.

Patients had to have more than 2 years of clinical follow-up to be included. Clinical progression was defined as the time patients first experienced either a new AIDS diagnosis (recurrences did not define an event) or death after baseline. Patients who experienced more than one AIDS diagnosis contributed only once to the analysis. The association between accumulation of resistance and risk of new AIDS and death was evaluated using standard survival analysis techniques (Kaplan–Meier curves and Cox proportional hazards regression model stratified by clinical centre), censoring patients' follow-up at the date of their last clinical visit. Group A was chosen as the comparator group in all analyses. The risk of progression in group B was also estimated and retained in the main analysis as a ‘control group’ for groups A and C. Group C was further divided into subgroups in various analyses using different definitions of drug resistance. These include drug class resistance mutations – a patient was defined to have drug class resistance mutations if at least one mutation associated with the drug class in question according to the 2006 update of the International AIDS Society (IAS) list [24] was detected; number of drugs predicted to be active by Rega interpretation system (; rules of version 7.1 for drugs currently used in clinical practice and version 6.4.1 for the remaining drugs – e.g. protease inhibitor used without ritonavir-boosting, etc.) among those received at baseline; and number of drugs predicted to be active by Rega interpretation system among those licensed by the European Medicines Agency (EMEA) by the calendar date of baseline (e.g. an estimate of the actual number of options available at the time of baseline). Patients in group C in whom no amino-acid changes associated with resistance according to the IAS list could be detected by the genotypic test were also evaluated as a separate exposure group. Further analyses were carried out using all patients but counting only major IAS protease inhibitor mutations to define protease inhibitor resistance. Finally, virological failure to a drug class was defined as a history of a viral load of more than 400 copies/ml 6 months after starting at least one drug of the class and when still receiving that class. All analyses were repeated using the separate endpoints of AIDS and death. Sensitivity analyses have been performed in specific subgroups (see ‘Results’ section).

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We studied 8229 patients in EuroSIDA who started cART and who had at least 2 years of clinical follow-up. At the date of initiation of their first cART, patients' median age was 37 (range:18–84) and 29% (n = 2423) had a previous AIDS diagnosis. The percentage of people who have used nucleosides before starting cART was 60% (n = 4964) and, in these, the median number of antiretrovirals used was 2 [interquartile range (IQR): 2–3]. Median (IQR) CD4 cell count (in 7122 patients with an available measurement) and viral load (in n = 5322) were 188 cells/μl (80–317) and 4.62 (3.89–5.21) log10 copies/ml, respectively. On average, the year of first cART initiation was 1997 (range:1996–2005).

Table 1 shows in more detail the characteristics of the population at baseline, stratified according to the groups A, B and C (Fig. 1). Patients in A, B and C were similar with regard to basic demographics and current use of antiretroviral regimens. However, as expected, patients who experienced virological failure over the first 2 years of cART had previously used, on average, a larger number of antiretrovirals, had a history of more extensive virological failure, lower latest CD4 cell count and a higher percentage of people with a previous AIDS diagnosis. Calendar year of baseline for patients in group C was, on average 2 years earlier compared with that of patients in group A.

Figure 2 shows the proportion of the first 2 years of follow-up spent on specific antiretrovirals stratified by group. Patients had been for the majority of time exposed to nucleosides (lamivudine and thymidine analogues) and protease inhibitor (mainly indinavir) with no appreciable differences between the groups.

Table 1 also shows the prevalence of resistance detected in the time window 0.5–2 years after cART initiation in patients in group C, both in terms of prevalence of specific mutations (or family of mutations) and according to an interpretation of their genotypic results (using the Rega interpretation systems, v6.4.1 and 7.1). In a minority of patients, prevalence was calculated cumulating the results obtained in two tests (n = 144, 15%) or at least three tests (n = 38, 4%). The majority of tests used in this analysis (91%, n = 885) were performed in the central laboratory.

The percentage of patients with double-class and triple-class resistance mutations was 58 and 21%, respectively. In more than one-third of the patients in group C, less than one drug included in the regimen received at baseline was predicted to retain activity against the patients' virus and 63% of the patients had less than seven drugs, among all those available at that specific point in time, which were predicted to be fully active.

There were 829 new AIDS events and 571 deaths during 38 814 person-years of follow-up (PYFU) for an overall crude incidence of new AIDS and death of 3.6 per 100 PYFU [95% confidence interval (CI): 3.4–3.8]. The Kaplan–Meier estimates of the overall cumulative proportion of people with a new AIDS diagnosis or death were 4.6% (95% CI: 4.1–5.0) by 1 year, 16.0% (95% CI: 15.2–16.8) by 5 years and 25.3% (95% CI: 23.7–26.9) by 8 years.

Figure 3a shows the Kaplan–Meier estimates stratified by groups (A, B and C); by 96 months from baseline, the proportion of patients with a new AIDS diagnosis or death was 20.3% (95% CI: 17.7–22.9) in group A, 30.4% (95% CI: 27.8–33.0) in B and 35.3% (95% CI: 31.2–39.4) in C (log-rank test P = 0.0001). This analysis shows a clear difference in risk of new AIDS and death when comparing patients with and without evidence of virological failure. This was confirmed in the multivariable analysis adjusted for potential confounders (relative hazard of group C vs. group A: 1.46, 95% CI: 1.18–1.80, P = 0.0005 and relative hazard of group B vs. group A: 1.40, 95% CI: 1.17–1.67, P = 0.0002). Within group C, patients who showed the highest risk of new AIDS and death were those who both rebounded and were tested while still on treatment (relative hazard vs. group A: 1.33, 95% CI: 1.04–1.70, P = 0.02).

Figure 3b shows the estimate comparing again patients in group A with those in group C who were, in turn, categorized according to the level of drug class resistance mutations detected by baseline (no resistance, single-class, double-class and triple-class resistance mutations). Within group C, patients with triple-class resistance mutations were those with the highest risk of new AIDS and death and there was a significant difference between groups (log-rank P = 0.0001). Table 2 shows the multivariable analysis corresponding to the Kaplan–Meier curves shown in Fig. 3b as well as a number of other models using alternative definitions of resistance described in the ‘Methods’. This analysis carried strong evidence that, even after adjusting for a large number of potential confounders, including CD4 cell count at baseline and calendar year of baseline, patients who accumulated resistance mutations to two or more drug classes over the first 2 years of cART were at higher risk of subsequently experiencing AIDS or death than those with no evidence of virological failure. Of note, when looking at specific drug classes, our analysis did not identify a particular drug class driving the association between accumulation of resistance and clinical outcome (Table 2).

Interestingly, the association between the virus predicted susceptibility and the risk of new AIDS and death was less strong than that observed with our simple definition of triple-class resistance. Compared with patients with no evidence of viral failure, the difference was significant for those receiving at baseline a regimen including less than one active drug and for those who had less than seven active drugs that could be used in future regimens.

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Sensitivity analyses

The estimate of the risk of new AIDS and death associated with the detection of triple-class resistance mutations as compared with no evidence of virological failure was consistent in a number of sensitivity analyses: relative hazard was 2.27 (95% CI: 1.56–3.30, P = 0.0001) when using only genotypic tests performed retrospectively in the central EuroSIDA laboratory, relative hazard, 1.46 (95% CI: 0.44–4.90, P = 0.54) in patients who started cART when they were ART-naive (n = 2712 patients and 245 clinical events), relative hazard, 1.33 (95% CI: 0.42–4.23, P = 0.63) in those who initiated cART with two nucleosides with either a NNRTI or a protease inhibitor/r (n = 1629, 136 events) and relative hazard, 1.43 (95% CI: 0.83–2.47, P = 0.20) in those who started when they were already under active follow-up in EuroSIDA (n = 1700, 284 events).

Patients with triple-class resistance mutations were at increased risk of new AIDS alone (relative hazard: 2.27, P = 0.002) compared with patients without virological failure. Consistent results were found for the endpoint death alone, although the power of this analysis was low (data not shown).

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The relationship between the emergence of HIV drug resistance in patients on ART and the risk of clinical outcome has been the subject of various analyses from cohort studies often using differing approaches [7–15]. Potentially as a result of the use of different methods of analysis as well as other factors, the results of these studies have been somewhat conflicting.

Our study provides strong evidence for an association between HIV drug resistance emerging on treatment and subsequent risk of new AIDS and death. The main strength of this analysis is the use of a large and well characterized cohort of patients with both genotypic resistance data and long-term prospective clinical follow-up.

The issue of trying to establish whether there is an association between emergence of HIV drug resistance and risk of clinical progression is complicated by several factors. First, the fact that resistance can only, in general, be measured using standard assays in people with viral load above a certain cut-off (typically ≥500 copies/ml). As a consequence of this, people who are tested for resistance on treatment in clinical cohort studies are potentially per se a selected population of patients at higher risk of disease progression compared, for example, with those with sustained virological suppression, regardless of whether drug resistance has been selected by ART or not. This introduces a limitation in that results can only be generalized to the population of patients routinely tested for resistance who may be not representative of all patients enrolled in the clinical cohort. Second, the lack of detectable resistance in a person experiencing virological failure on therapy is typically a marker of patients' poor adherence, which, in turn is associated with poor clinical outcome. Previous analyses [7,8] compared the risk of clinical progression between people with virological failure and no detectable resistance mutations (patients likely to be poorly adherent) with that of patients with evidence of resistance at virological failure (patients with reduced benefit from therapy due to resistance). The interpretation of such analysis is not straightforward, as it seems difficult to predict a priori which of these two groups may experience, in the long run, the worst clinical outcome. In contrast, our alternative analysis design allowed the inclusion, and the use as the comparator group, of patients with undetectable viral load. Interestingly, our analysis carried evidence that patients in whom no resistance mutations could be detected at failure (4% of tested) tended to be at increased risk of new AIDS and death compared with patients without virological failure, suggesting that poor adherence may have a long-term detrimental effect on the clinical outcome. However, of course, poor adherence cannot be assumed to be the only explanation for treatment failure in the presence of viral rebound with no detection of resistance and in our cohort, in which the majority of patients started ‘old generation’ antiretrovirals, issues such as drug potency and cellular resistance may have played a role [25].

Furthermore, there is the issue of which statistical approach may be best to answer this clinical question; viral load is likely to be both a time-dependent confounder and an intermediate factor for the relationship between accumulation of resistance and clinical outcome [26–32]. In such a situation, marginal structural modelling (MSM) [33] is preferable to standard survival analysis with time-dependent covariates [15] if the aim is to estimate the true magnitude of the association of interest. However, because resistance can be measured only in patients with raised viral load and because current assays can only detect mutations present in the dominant virus, it is likely that the analysis of the determinants of drug resistance may produce unreliable inverse probabilities of weighting. Also, although MSM is ideally suited to estimate the net effect of an intervention (typically the initiation of treatment) on the outcome, testing for resistance it is not an intervention per se but rather a tool that is used to guide treatment interventions and therefore the causal question is not well defined [20,21,33].

Before drawing conclusions, some possible limitations of our analysis need to be mentioned. First, the fact that resistance to drug classes was based on the sequencing of reverse transcriptase and encoding protease genomes alone so that for people with resistance in other regions of HIV (e.g. gag) or to newly approved classes of drugs (fusion-inhibitor, integrase-inhibitors and CCR5 antagonists) we may have underestimated the number of drug classes to which their virus populations were resistant. However, only eight patients were exposed to T20 before baseline and none to the other newer drugs. Second, the majority of the genotypic tests were performed retrospectively on stored plasma samples and Group C may have not been representative of the whole population of viraemic patients in EuroSIDA. Nevertheless, we did not find major differences in their risk of new AIDS and death comparing patients in Group C (with genotypic test) with those of group B (without genotypic test, Fig. 3a). Third, although we used multiple genotypic tests for some individuals, sensitivity of current routine genotypic assays is low so that the real level of resistance may have been underestimated. However, if anything, as a consequence of this bias, our estimate of the difference in risk of clinical progression between groups may have also been underestimated. Finally, ours is an historical analysis including mainly patients who started cART before the year 2000 and results may not be generalized to patients starting cART more recently who may have the benefit of experiencing the newer classes of drugs. However, the estimate of the association between detection of triple-class resistance and risk of new AIDS and death in patients who started regimens recommended by current guidelines was consistent with that of our main analysis, though not statistically significant. This lack of significance is likely to be due to low statistical power of the analysis as a consequence of the low incidence of triple-class resistance mutations in this subgroup of patients. This assessment should, therefore, be repeated in the future as data of patients starting cART with currently recommended regimens will accumulate.

Our analysis shows evidence that the detection of resistance in patients on therapy is associated with their long-term clinical outcome. In particular, our data show that there is an 55% increase in the risk of new AIDS and death in patients who, by 2 years of starting their first cART, accumulated resistance to two classes of antiretrovirals and an 80% increase in those who accumulated resistance to nucleosides, NNRTI and protease inhibitor. Because we only studied baseline factors, the potential mechanisms behind this association need to be further investigated. Adherence may be one of the likely explanations of our findings and we cannot rule out residual confounding by health-seeking behaviour, quality of care or other so far unrecognized factors.

These findings are consistent with those of other studies [7–11] and extend the results of another study in which a more stringent definition of class resistance was used [7]. Our analysis, however, does not support the previous observation that emergence of resistance to NNRTI is associated with a greater risk of subsequent death than the emergence of resistance to nucleosides or protease inhibitor [14,15]. Of note, the NNRTI-specific effect on clinical outcome in these other studies was found using MSM [14] or standard survival analysis with time-dependent covariates [15] and could be also reproduced in EuroSIDA using these methods (data not shown). Further work is required to identify possible causes for this discrepancy.

In conclusion, our findings further support the view that experiencing early virological failure with drug resistance is a prognostic sign for poorer long-term clinical outcome. This is likely to be due to both a tendency for those with drug resistance to be poorly adherent (and therefore patients' education about the risk of resistance associated with less than perfect adherence should be reinforced) and a direct effect of resistance on subsequent outcome; further research is needed to separate the effect of these two distinct potential mechanisms.

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Primary support for EuroSIDA is provided by the European Commission BIOMED 1 (CT94-1637), BIOMED 2 (CT97-2713), the 5th Framework (QLK2-2000-00773) and the 6th Framework (LSHP-CT-2006-018632) programs. Current support also includes unrestricted grants by Bristol-Myers Squibb, GlaxoSmithKline, Roche, Gilead, Pfizer, Merck and Co., Tibotec and Boehringer-Ingelheim. The participation of centres from Switzerland was supported by a grant from the Swiss Federal Office for Education and Science.

Over the past few years, some of the authors have received reimbursement, fees and/or funding for attending symposiums, speaking, advisory board membership, organising educational activities, consulting and/or research from Abbott (A.N.P., J.D.L.), Boeringher Ingelheim (A.N.P., J.D.L.), Bristol-Myers Squibb (A.N.P., J.D.L., A.M.), Gilead Sciences (A.N.P., J.D.L.), GlaxoSmithKline (A.C.-L., A.N.P., J.D.L.), Pfizer Pharmaceutical (A.N.P., J.D.L.), Roche (A.C.-L., A.N.P., J.D.L.), and Tibotec (A.N.P., J.D.L.). None of the other authors have declared a conflict of interest. None of the authors hold any shares in any of the companies.

Presented at 10th European AIDS Conference/EACS; Dublin 17–20 November 2005; Abstract PS6/4.

Role of each author

Design of the study (J.D.L., O.K., A.M., A.N.P.)

Data collection (J.D.L., O.K., A.L., A.W.-D., A.K.)

Analysis design (A.C.-L., A.N.P.)

Statistical analysis of the data (A.C.-L.)

Genotypic testing (B.C., L.R.)

Contribution to the writing of the paper (A.C.-L., A.N.P., B.C., A.M., L.R., O.K., A.L., A.W.-D., A.K. and J.D.L.)

The EuroSIDA Study Group

The names of the national coordinators are given in parenthesis.

(M. Losso), A. Duran, Hospital JM Ramos Mejia, Buenos Aires, Argentina.

(N. Vetter), Pulmologisches Zentrum der Stadt Wien, Vienna, Austria.

(I. Karpov), A. Vassilenko, Belarus State Medical University, Minsk, Belarus.

(N. Clumeck), S. De Wit, B. Poll, Saint-Pierre Hospital, Brussels; R. Colebunders, Institute of Tropical Medicine, Antwerp, Belgium.

K. Kostov, Infectious Diseases Hospital, Sofia, Bulgaria.

J. Begovac, University Hospital of Infectious Diseases, Zagreb, Croatia.

(M. Ristola), Helsinki University Central Hospital, Helsinki, Finland.

(L. Machala), H. Rozsypal, Faculty Hospital Bulovka, Prague; D. Sedlacek, Charles University Hospital, Plzen, Czech Republic.

(J. Nielsen), J. Lundgren, T. Benfield, O. Kirk, Panum Institute, Copenhagen; J. Gerstoft, T. Katzenstein, A.-B. E. Hansen, P. Skinhøj, Rigshospitalet, Copenhagen; C. Pedersen, Odense University Hospital, Odense; L. Oestergaard, Skejby Hospital, Aarhus, Denmark.

(K. Zilmer), West-Tallinn Central Hospital, Tallinn; J. Smidt, Nakkusosakond Siseklinik, Kohtla-Järve, Estonia.

(C. Katlama), Hôpital de la Pitié-Salpétière, Paris; J.-P. Viard, Hôpital Necker-Enfants Malades, Paris; P.-M. Girard, Hospital Saint-Antoine, Paris; J.M. Livrozet, Hôpital Edouard Herriot, Lyon; P. Vanhems, University Claude Bernard, Lyon; C. Pradier, Hôpital de l'Archet, Nice; F. Dabis, Unité INSERM, Bordeaux, France.

(J. Rockstroh), Universitäts Klinik Bonn; R. Schmidt, Medizinische Hochschule Hannover; J. van Lunzen, O. Degen, University Medical Center Hamburg-Eppendorf, Infectious Diseases Unit, Hamburg; H.J. Stellbrink, IPM Study Center, Hamburg; S. Staszewski, JW Goethe University Hospital, Frankfurt; J. Bogner, Medizinische Poliklinik, Munich; G. Fätkenheuer, Universität Köln, Cologne, Germany.

(J. Kosmidis), P. Gargalianos, G. Xylomenos, J. Perdios, Athens General Hospital; G. Panos, A. Filandras, E. Karabatsaki, 1st IKA Hospital; H. Sambattakou, Ippokration Genereal Hospital, Athens, Greece.

(D. Banhegyi), Szent Lásló Hospital, Budapest, Hungary.

(F. Mulcahy), St. James's Hospital, Dublin, Ireland.

(I. Yust), D. Turner, M. Burke, Ichilov Hospital, Tel Aviv; S. Pollack, G. Hassoun, Rambam Medical Center, Haifa; S. Maayan, Hadassah University Hospital, Jerusalem, Israel.

(A. Chiesi), Istituto Superiore di Sanità, Rome; R. Esposito, I. Mazeu, C. Mussini, Università Modena, Modena; C. Arici, Ospedale Riuniti, Bergamo; R. Pristera, Ospedale Generale Regionale, Bolzano; F. Mazzotta, A. Gabbuti, Ospedale S Maria Annunziata, Firenze; V. Vullo, M. Lichtner, University di Roma la Sapienza, Rome; A. Chirianni, E. Montesarchio, M. Gargiulo, Presidio Ospedaliero AD Cotugno, Monaldi Hospital, Napoli; G. Antonucci, F. Iacomi, P. Narciso, C. Vlassi, M. Zaccarelli, Istituto Nazionale Malattie Infettive Lazzaro Spallanzani, Rome; A. Lazzarin, R. Finazzi, Ospedale San Raffaele, Milan; M. Galli, A. Ridolfo, Ospedale L. Sacco, Milan; A. d'Arminio Monforte, Istituto Di Clinica Malattie Infettive e Tropicale, Milan, Italy.

(B. Rozentale), P. Aldins, Infectology Centre of Latvia, Riga, Latvia.

(S. Chaplinskas), Lithuanian AIDS Centre, Vilnius, Lithuania.

(R. Hemmer), T. Staub, Centre Hospitalier, Luxembourg, Luxembourg.

(P. Reiss), Academisch Medisch Centrum bij de Universiteit van Amsterdam, Amsterdam, Netherlands.

(J. Bruun) A. Maeland, V. Ormaasen, Ullevål Hospital, Oslo, Norway.

(B. Knysz), J. Gasiorowski, Medical University, Wroclaw; A. Horban, Centrum Diagnostyki i Terapii AIDS, Warsaw; D. Prokopowicz, A. Wiercinska-Drapalo, Medical University, Bialystok; A. Boron-Kaczmarska, M. Pynka, Medical Univesity, Szczecin; M. Beniowski, E. Mularska, Osrodek Diagnostyki i Terapii AIDS, Chorzow; H. Trocha, Medical University, Gdansk, Poland.

(F. Antunes), E. Valadas, Hospital Santa Maria, Lisbon; K. Mansinho, Hospital de Egas Moniz, Lisbon; F. Maltez, Hospital Curry Cabral, Lisbon.Romania: (D. Duiculescu), Spitalul de Boli Infectioase si Tropicale: Dr Victor Babes, Bucarest, Portugal.

(A. Rakhmanova), Medical Academy Botkin Hospital, St Petersburg; E. Vinogradova, St Petersburg AIDS Centre, St Peterburg; S. Buzunova, Novgorod Centre for AIDS, Novgorod, Russia.

(D. Jevtovic), The Institute for Infectious and Tropical Diseases, Belgrade, Serbia.

(M. Mokráš), D. Staneková, Dérer Hospital, Bratislava, Slovakia.

(J. González-Lahoz), V. Soriano, L. Martin-Carbonero, P. Labarga, Hospital Carlos III, Madrid; B. Clotet, A. Jou, J. Conejero, C. Tural, Hospital Germans Trias i Pujol, Badalona; J.M. Gatell, J.M. Miró, Hospital Clinic i Provincial, Barcelona; P. Domingo, M. Gutierrez, G. Mateo, M.A. Sambeat, Hospital Sant Pau, Barcelona, Spain.

(A. Karlsson), Karolinska University Hospital, Stockholm; P.O. Persson, Karolinska University Hospital, Huddinge; L. Flamholc, Malmö University Hospital, Malmö, Sweden.

(B. Ledergerber), R. Weber, University Hospital, Zürich; P. Francioli, M. Cavassini, Centre Hospitalier Universitaire Vaudois, Lausanne; B. Hirschel, E. Boffi, Hospital Cantonal Universitaire de Geneve, Geneve; H. Furrer, Inselspital Bern, Bern; M. Battegay, L. Elzi, University Hospital Basel, Switzerland.

(E. Kravchenko), N. Chentsova, Kiev Centre for AIDS, Kiev, Ukraine.

(S. Barton), St. Stephen's Clinic, Chelsea and Westminster Hospital, London; A.M. Johnson, D. Mercey, Royal Free and University College London Medical School, London (University College Campus); A. Phillips, M.A. Johnson, A. Mocroft, Royal Free and University College Medical School, London (Royal Free Campus); M. Murphy, Medical College of Saint Bartholomew's Hospital, London; J. Weber, G. Scullard, Imperial College School of Medicine at St. Mary's, London; M. Fisher, Royal Sussex County Hospital, Brighton; R. Brettle, Western General Hospital, Edinburgh, United Kingdom.

Virology group: B. Clotet (central coordinator) plus ad hoc virologists from participating sites in the EuroSIDA Study.

Steering committee: F. Antunes, B. Clotet, D. Duiculescu, J. Gatell, B. Gazzard, A. Horban, A. Karlsson, C. Katlama, B. Ledergerber (chair), A. d'Arminio Monforte, A. Phillips, A. Rakhmanova, P. Reiss (vice-chair), J. Rockstroh.

Coordinating centre staff: J. Lundgren (project leader), O. Kirk, A. Mocroft, N. Friis- Møller, A. Cozzi-Lepri, W. Bannister, M. Ellefson, A. Borch, D. Podlevkareva, J. Kjær, L. Peters, J. Reekie

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clinical progression; drug resistance mutations; virological failure

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