Predictive value of the HIV paediatric classification system for the long-term course of perinatally infected children

Italian Register for HIV Infection in Children Writing Committee:L Gallia, M de Martinoa, P-A Tovob, C Gabianoa and M Zappac

a Department of Paediatrics, University of Florence, Florence, Italy.
b Department of Paediatrics, University of Turin, Turin, Italy.
c Epidemiology Unit, Centre for the Study and Prevention of Cancer, Careggi Hospital, Florence, Italy.

Reprint requests to: Prof. Maurizio de Martino, Coordinator of the Italian Register for HIV Infection in Children, Department of Paediatrics, University of Florence, Via Luca Giordano 13, I-50132 Florence, Italy. E-mail: mdm{at}ao-meyer.toscana.it


    Abstract
 Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
 Appendix
 References
 
Background To compare the Centers for Disease Control and Prevention (CDC) paediatric classification system with the long-term course of perinatal human immunodeficiency virus type 1 (HIV-1) infection.

Methods Prospective study on 366 perinatally infected children followed-up from birth and checked at least every 2 months. Survival, smoothed hazard, adjusted hazard ratio of death, and transition probabilities in clinical and immunological categories were outcome measures.

Results Survival was 49% (95% CI : 40–58%) at 8 years. The risk of death was high before the age of 2, relatively low between ages 2 and 7, and contained thereafter. Children did not advance through the categories sequentially. Survival at 8 years was 61.7% (95% CI : 49.8–73.6%) in those children who had passed through clinical category A; the hazard ratio of death was 2.5 (95% CI : 1.7–3.8) for 175 (47.9%) children who skipped this category. Transition probability in clinical category B was 39.9% (95% CI : 32.3–45.6%) after one year, but 59.1% (95% CI : 51.4–66.8%) after 5 years. Before 2 years of age, the probability of entry into category C (40%; 95% CI : 35–45%) was higher than that of entry into immunological category 3 (28%; 95% CI : 22–34%).

Conclusions The classification system stands comparison with the clinical reality, but the CD4-positive cell thresholds in infancy should be adjusted and category B indicator diseases better distributed to improve their predictive value.

Keywords CD4-positive lymphocytes, CDC paediatric classification system, disease progression, perinatal HIV-1 infection, survival

Accepted 27 October 1999


    Introduction
 Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
 Appendix
 References
 
Two prospective studies, the European Collaborative Study together with the French Paediatric Human Immunodeficiency Virus (HIV) Infection Study1 and the Women and Infants Transmission Study (WITS),2 have recently described the clinical course of HIV-1 perinatal infection. These studies raised the question of the suitability of the 1994 Centers for Disease Control and Prevention (CDC) classification3 in being able to adequately fit the clinical course and predict disease progression. Validation is of interest as the system is aimed at reflecting the stage of infection and providing predictive information.3 The present study describes the actual long-term course of the disease within the prospective cohort of the Italian Register for HIV Infection in Children, and contributes towards the validation of the classification system.


    Patients and Methods
 Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
 Appendix
 References
 
Italian Register for HIV Infection in Children
The Italian Register for HIV Infection in Children is a nationwide multicentre study on HIV-1 perinatally exposed children, which was instituted in 1985 by the Italian Association of Paediatrics.4–8 The Register involves a network of 103 paediatric centres which participate voluntarily and forward to two co-ordinating centres at the Departments of Paediatrics in Florence and Turin. The centres undertake to enrol all exposed children. Up to 30 June 1997, 2252 children (366 definitely infected, 440 of still indeterminate infection status, and 1446 seroreverted uninfected) born to HIV-1 infected mothers and followed-up prospectively from birth were enrolled. The data set is representative of the overall population of exposed children in Italy.5

Data are collected through registration and follow-up forms which contain questions on child's demographic data, age at first observation, maternal data (including clinical condition at the time of delivery), perinatal data (including gestational age, birthweight, and type of infant feeding), HIV-1 antibodies, virus markers (proviral DNA, virus culture, free and complexed p24 antigenaemia), CDC classification, CD4-positive cell numbers (measured by standardized fluorescent-activated cell sorting technique),4 laboratory tests, HIV-1 related signs and age at the appearance of single signs, the age at the entrance to the clinical and immunological categories of the CDC paediatric classification system or at death, Pneumocystis carinii pneumonia (PCP) chemoprophylaxis, drug used for PCP chemoprophylaxis, date at the beginning and end of chemoprophylaxis with each drug, antiretroviral therapy (ART) and drug(s) used, date at the beginning and end of therapy with each antiretroviral drug, date at last check-up or at death (with causes). Forms are completed every six months by the appointed paediatrician at each centre.7 Quality checks are carried out in duplicate on all forms by the two co-ordinating centres before any data are entered into specific software. Further checks are subsequently carried out on all data included in the database. Paediatricians from participating centres meet at least once a year to audit proceedings and standardize procedures. According to these, all infected children (independently of their clinical condition) are examined both clinically and immunologically at least every 2 months, and the exact date of any clinical and of any confirmed immunological changes are reported in the follow-up form.

Case definition
The whole cohort of 366 children definitely infected and enrolled before 30 June 1997, were studied. Infection was diagnosed through detection (on at least two occasions) of virus markers, or the persistence of HIV-1 antibodies after 18 months of life, or the onset of AIDS-defining signs.3 The CDC classification system defined the clinical and immunological condition.3 This system provides mutually exclusive states based on the intersection of four clinical and three immunological categories. Clinical categories classify children with no (category N), mild (category A), moderate (category B), or severe (category C) AIDS-defining signs. Immunological categories are based on thresholds of CD4-positive cell numbers adjusted by age and distinguish no (category 1), moderate (category 2) and severe (category 3) immunosuppression. The criteria for HIV-1 related disease used in defining the CDC clinical categories have been previously reported.6–8

The status of each subject at this analysis was the one attributed at the date of last clinical check or death. Only HIV-1 related deaths were considered when survival was estimated;6–8 subjects whose death was not directly attributable to HIV-1 infection were censored at last check. No standardized criteria exist to define long-term non-progressors (LTNP). We defined as LTNP those children surviving longer than the median survival and classified as N1 or A1 (category N and A signs do not predict negatively).6–8 The CDC classification for HIV-1 infection in adults was used in defining mothers' clinical condition at the time of delivery.9

Statistical analyses
Data were processed by SPSSX (SPSS Inc., Chicago, IL) and EGRET (SERC Software Division, Washington, DC) statistical packages. Ages were reported as median and range. The transition probability (i.e. the cumulative probability of moving from one category to a worse one or death within one and 5 years) was calculated by the Kaplan-Meier product-limit method; the 95% CI were estimated on the basis of Greenwood estimate of variance using a transformation to avoid impossible values.10,11 Differences in curves were tested by the Log-rank test. To describe the pattern of hazard of dying over time, smoothed hazard curves (according to the Kernel function) were plotted with bandwidths of 12 months.12 Cox proportional hazard methods were used to evaluate factors independently associ-ated with survival and the clinical course. Covariates entered into the model were gender, year of birth (as a continous variable), gestational age (<36 weeks versus >36 weeks), birthweight (<2400 g versus >2400 g), mother's clinical condition at the time of delivery (asymptomatic category A [276] versus symptomatic category B [51] and C [4] women), type of infant feeding (ever breastfed versus exclusively formula-fed) and treatments (no treatment, PCP chemoprophylaxis, ART, PCP chemoprophylaxis and ART). First, all covariates were adjusted for each other. Then, the variables were selected according to ‘backward elimination procedure’.13 In assessing the transition probability from a specific category, only treatments received after entry into that category and before passage to a worse one or death were considered (to avoid the bias of treatments given as a consequence of a worsening in the clinical or immunological course). To define history in infancy, children aged <18 months at the time of analysis (33) were also included. Since some centres in our Register did not have facilities to routinely carry out PCR or viral culture, it was possible that some severely affected infants <18 months were identified as HIV-1 infected and entered the study, whereas other asymptomatic age-matched infants were not identified as HIV-1 infected and did not enter the study. Because this could bias the results, survival was compared in children aged < or >18 months and probability of entering category C and survival were calculated also excluding the former children. Two (i.e. the end of the first peak of mortality in the bimodal course of perinatal infection)6–8 and 8 years of age (end of our follow-up) were chosen as ages to compare survival and timing of entry into clinical and immunological categories.


    Results
 Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
 Appendix
 References
 
Description of the study group
In all, 366 children (185 females, 181 males, median age 33 months [range: 0.7–154]) were studied. No sets of twins but seven pairs of siblings were included (a preliminary analysis had shown that their inclusion did not bias the results). Characteristics of the study group are reported in Table 1Go. The PCP chemoprophylaxis (oral trimethoprim-sulphamethoxazole or aerosolized pentamidine isethionate) was initiated at median age 12.9 months (range: 0.3–103.9) for a median duration of 21.2 months (range: 0.03–74.5). In children receiving ART, zidovudine (ZDV) was administered first in 202 cases, whereas two children started treatment with didanosine (ddI). Didanosine was subsequently added in six children, whereas ddI replaced ZDV in 26. Antiretroviral therapy was started at median age 11.9 months (range: 1.01–113.4) and the median duration was 27.4 months (range: 0.03–81.9). At this analysis, no child received a highly active antiretroviral therapy (HAART).14 In all, 113 children died (one of non HIV-1 related disease) and 27 (7.3%) children were lost to follow-up at a median age 22 months (range: 0.7–83). Clinical categories at last check of children lost to follow-up were: N = 6 (22.2%); A = 4 (14.8%); B = 13 (48.1%); C = 4 (48.6%).


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Table 1 Characteristics of the study group
 
Passage through clinical and immunological categories
All children but one (0.27%) were in category N and all were in category 1 at birth. Table 2Go reports the number of children entering in each category and the transition probability after one and 5 years. Children passed rapidly through category N. More then 40% of children who entered category B were still in this category after 5 years. Passages through the immunological categories were more gradual than those through clinical categories, but the category 3 showed the lowest transition probability. Category A was not a necessary step: 142 (38.7%) children entered category B directly at median age 4.8 months (range: 0–46) and 33 (9.1%) entered directly into category C at median age 3.7 months (range: 1–61). Up to 3 years of age the probability of entry into category C was higher than that of entry into category 3 (Figure 1Go). For the entire cohort the probability of entry into category C was 40% (95% CI : 35–45%) at 2 years old and 62% (95% CI : 54–70%) at 8 years of age, whereas the probability of entry into category 3 was 28% (95% CI : 23–34%) and 67% (95% CI : 60–74%), respectively. Excluding children aged <18 months at the time of analysis, the probability of entry into category C was 39% (95% CI : 33– 45%) at 2 years old and 62% (95% CI : 53–71%) at 8 years of age. Four children fitted LTNP definition. The probability of becoming LTNP was 2.9% (95% CI : 0.6–5.8%).


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Table 2 Transition probability (cumulative probability of moving from one category to a worse one or death and 95% CI) one and 5 years after entry in each clinical and immunological category; the number of children entering each category is indicated
 


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Figure 1 Probability of entry into clinical category C and immunological category 3

 
Survival
The survival probability dropped to 79% (95% CI : 74–83%) at 2 and reached 49% (95% CI : 40–58%) at 8 years of age. Smoothed hazard of death (Figure 2Go) defined a peak of risk during the first 2 years of life, a subsequent low-risk period until approximately 7 years; subsequently a second peak appeared. Inclusion in the study group of children aged <18 months at the time of analysis did not bias the results since their estimated survival at 18 months was similar to that of children aged >18 months at the time of analysis (85% [95% CI : 81–88%] versus 84% (95% CI : 80–88%]). Excluding children aged <18 months, survival was 79% (95% CI : 73–84%) at 2 and 49% (95% CI : 39–59%) at 8 years of age, similar to that observed in the whole study group.



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Figure 2 Smoothed hazard of death in children with perinatal human immunodeficiency virus type 1 (HIV-1) infection. The ratio between the number of events (deaths) and number of children still alive at each birthday is reported

 
Figure 3Go shows survival after entry into each clinical (Figure 3aGo) and immunological (Figure 3bGo) category. Category A selected children had the best outcome. Their survival at 8 years of age was 61.7% (95% CI : 49.8–73.6%) versus 40.7% (95% CI : 26.1–53.4%) in 176 children who skipped category A. The adjusted hazard ratio for death was 2.5 (95% CI : 1.7–3.8; P < 0.001) for the latter group. Any advantage disappeared when children switched from category A to category B: from this moment onwards, the adjusted hazard ratio for the death of children who entered category B, after switching from category A, was 0.97 (95% CI : 0.60–1.54; P = 0.864), as compared to those children who skipped category A. Survival 5 years after entry into category C did not differ (P = 0.14) in 122 children who developed category C-defining signs at age <18 months (17.5%; 95% CI : 10.1–25.4%) and in 42 children who developed these signs when aged >18 months (21.4%; 95% CI : 7.5–41.2%). Survival was significantly shorter (P < 0.001) in 77 children who entered category 3 at age <18 months (10.2%; 95% CI : 3.5–21.1%), as compared to 87 who entered thereafter (29.3%; 95% CI : 12.5–48.7%). Eighty-two children died before the age of 3 years: all but one (98.7%) in clinical category C but only 52 (63.4%) in immunological category 3.



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Figure 3 Survival probability after entry into each clinical (a) or immunological (b) category

 
Factors associated with changes in survival and clinical course
The adjusted hazard ratio of death was not significant, in the saturated model, when evaluated by gender (1.3 [95% CI : 0.9–1.9]; P = 0.45), birthweight (0.7 [95% CI : 0.4–1.3]; P = 0.28), gestational age (1.2 [95% CI : 0.7–2.3]; P = 0.47), mode of delivery (1.3 [95% CI : 0.8–2.1]; P = 0.28), maternal clinical condition at the time of delivery (0.9 [95% CI : 0.4–1.5]; P = 0.40), year of birth (1.1 [95% CI : 1.0–1.2]; P = 0.07), type of infant feeding (1.6 [95% CI : 0.7–3.4]; P = 0.28). Significantly reduced hazard ratios were associated, in the final model, with PCP chemoprophylaxis (0.2 [95% CI : 0.1–0.5]; P = 0.003), ART (0.6 [95% CI : 0.4–0.9]; P = 0.02), and combined PCP chemoprophylaxis and ART (0.3 [95% CI : 0.2–0.5]; P < 0.001). Transition probability in each category was lower when PCP chemoprophylaxis and/or ART was given and ART more than PCP chemoprophylaxis was associated with a lower transition probability in immunological categories (Table 3Go).


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Table 3 Transition probabilities (cumulative probability of moving from one category to a worse one or death and 95% CI) one and 5 years after entry in each clinical and immunological category according to treatments; the number of children entering each category is indicated
 

    Discussion
 Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
 Appendix
 References
 
The CDC classification system has been compared with the clinical reality. The first flaw in the classification system is the discrepancy between clinical condition and immunological classification in infancy, when clinical progression precedes the progression into immunological categories. This finding is in agreement with the WITS cohort results which suggest (describing a smaller data set with a shorter median follow-up, compared to the present) that clinical progression, as defined by the clinical axis of the CDC classification system, occurs in infancy faster than progression along the immunological axis.2 Since clinical and immunological checks at each centre were coincident, we can exclude that lack of synchrony between the clinical and immunological categories might depend on different timing in their assessment. Notably, the proportions of children who entered category C or category 3 within 2 years of life were similar in our and the WITS study.2 A revision of CD4-positive cell number thresholds in infancy could improve the predictive value of immunological categories, and major collaborative studies could contribute data on infants to define predictive thresholds.

The second downfall is a poor predictive significance for clinical category B, either a long-lasting stage of moderate disease (59.1% transition probability after a 5-year period in this category) or the threshold to AIDS (39.9% probability of progression to AIDS after one-year period). Category B indicator diseases are widely different in prognostic importance6–8 and distributing indicator diseases, according to criteria of predictivity, could make the classification system more suitable to clinical reality. A high-risk subcategory B could include anaemia, candidiasis, cardiomyopathy, diarrhoea, hepatitis, and persistent fever. We have previously shown that these category B signs are those significantly associated with a shorter survival.6–8

Different from the previously proposed Markov chain modelling,15 children do not advance through the categories in sequence and category A singles out a more favourable state than even category N, from which half of the children move directly into category B or AIDS. High transition probability in category N is the result of a short symptom-free period, due to the infants' immunological immaturity and permissiveness towards HIV-1.8 The category A signs resemble the adults' diffuse infiltrative lymphocytosis syndrome rather than the acute clinical syndrome, which correlates with short survival.7 Category C is highly discriminatory because it is the almost unique category at risk of death; 44.9% of category C children die within one year after entry in this category. Category 3 is less discriminatory (28.6% of category 3 children die within one year after entry in this category and only 63.4% of children die in this category before the age of 3 years) probably due to circulating CD4-positive cell numbers which are unrepresentative of the total body pool.7 The short survival of infants who rapidly enter category 3 confirms that dynamic data of CD4-positive cell loss, rather than static data, have a predictive meaning.4,7

Long-term non-progressors are a negligible portion of infected children, due also to the fact that subjects cannot be reclassified into a less severe category even if their clinical or immunological status improves.1,3 Nevertheless, children have a relatively slow progression besides some infants who rapidly develop AIDS and die probably because of virus transmission in utero, early altered immunity, and a high viral load.4,7,8,16 The risk of death slowed somewhat after 7 years of life, even though we must be careful due to the number of events and sample size at older ages. Due to these limitations a longer follow-up of our cohort is needed to confirm this pattern.

Differences in the outcome may be associated with year of birth,17 mothers' clinical condition at the time of delivery,18 birthweight,8 and type of infant feeding.19 In addition there is recent evidence that children infected in spite of maternal ZDV treatment in pregnancy have a particularly severe outcome.20 We did not consider maternal ZDV treatment as a covariate because the whole data set of children (born since 1984) was taken into account in the present study whereas children born to ZDV-treated mothers were born after 1992. Since the standard of care has been radically changed over time, any comparison between these two groups would be misleading.

The lack of significance of such factors in our analysis probably does not depend on their lack of importance. Rather, the strong association between the outcome and treatments overpowered any other statistical significance (only year of birth, which correlates with treatments,17 neared significance). The effectiveness of PCP chemoprophylaxis and the benefits of ZDV and ddI treatments in children have been demonstrated.21,22 The lower progression in immunological categories when ART is given is revealing, since it may be that ART slows down CD4-positive cell loss.22 However, the effects of treatment on survival, as observed in the present study, need to be considered with caution: some severely affected children may not have received treatment because of course rapidity (overestimating the effect) and some mildly affected children may not have received treatment because their clinical condition did not require it (underestimating the effect). Treatments could be also an indicator of factors (e.g. family compliance, socioeconomic status, attention and care devoted to the child) which may effect the outcome.23 It is unlikely that the small number of children lost to follow-up biased results since slight differences in clinical categories, as compared to the remaining study population, were representative of their younger age. Findings may be relevant in clinical management, family counselling, health policies, and trial design. The classification system supports the long-term clinical course satisfactorily, but may not predict the prognosis in all age groups and categories. The results of the present study are important also in that they incorporate treatments in multivariate analysis, along with other more traditionally recognized variables related to disease progression. The fact that the treatment variables overcome these other variables adds to the literature because previous studies have not had enough treated subjects to be able to establish the importance of treatment in relation to disease progression. The adoption of HAART at a progressively earlier stage in most children14,24 will probably bring about further radical changes in the long-term course of the infection. The present study aimed to give an outlook on the long-term course of perinatal HIV-1 infection and provide Italian data on the suitability of the 1994 CDC classification system, comparing the results with those of other large multicentre studies carried out in the pre-HAART era in France1 and USA.2

Any future analysis of outcome in perinatally HIV-1 infected children will be compared with the clinical course as outlined in the present study in children of untreated mothers and receiving pre-HAART regimens.


    Appendix
 Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
 Appendix
 References
 
Participants in the Italian Register for HIV Infection in Children
Ancona: P Osimani, MD; Bari: D De Mattia, MD, P Zizzadoro, MD; Bergamo: M Ruggeri, MD; Bologna: F Baldi, MD, M Ciccia, MD, P Dallacasa, MD, M Masi, MD; Bolzano: L Battisti, MD; Brescia: E Bresciani, MD, M Duse, MD, S Timpano, MD; Brindisi: PG Chiriacò, MD; Cagliari: M Belloni, MD, A Corrias, MD, P Ibba, MD, G Rossi, MD; Catanzaro: E Anastasio, MD; Chieti: G Sabatino, MD; Como: M Sticca, MD; Cuneo: C Nasi, MD; Ferrara: T Bezzi, MD; Florence: A Vierucci, MD, S Farina, MD, S Ballotti, MD; Genoa: D Bassetti, MD, A De Maria, MD, GL Forni, MD, C Gotta, MD, MG Marazzi, MD, D Mecca, MD, L Tasso MD; Imperia: U Tondo, MD; Leghorn: E Micheletti, MD; Mantova: G Gambaretto, MD; Modena: M Cellini, MD; Milan: R Altobelli, MD, A Bucceri, MD, S Conio, MD, G Ferraris, MD, M Giovannini, MD, R Lipreri, MD, P Marchisio, MD, E Massironi, MD, R Pinzani, MD, A Plebani, MD, L Rancilio, MD, E Riva, MD, F Salvini, MD, R Tornaghi, MD, GV Zuccotti, MD; Naples: A Guarino, MD, C Pignata, MD; Padua: C Giaquinto, MD, O Rampon, MD, EM Ruga MD; Palermo: A Romano, MD; Parma: G Benaglia, MD; Pavia: D Caselli, MD, A Maccabruni, MD; Piacenza: F Bassanetti, MD; Pisa: R Consolini, MD, G Palla, MD; Ravenna: A Antonellini, MD, AM Metri, MD; Reggio Emilia: C Magnani, MD; Rimini: MT Cecchi, MD; Rome: G Castelli Gattinara, MD, S Catania, MD, P Falconieri, MD, C Fundarò, MD, O Genovese, MD, A Krzisztofiak, MD, S Livadiotti, MD, C Rendeli, MD, M Stegagno, MD, C Timpano MD; Trento: A Mazza, MD; Trieste: CM Salvatore, MD; Turin: E Palomba, MD, C Riva, MD, S Tulisso, MD; Varese: A Pellegatta, MD.


    Acknowledgments
 
This work was supported by the Italian Ministero della Sanità, Istituto Superiore di Sanità, Progetto AIDS 1998. The authors thank the two anonymous referees who provided helpful comments.


    References
 Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
 Appendix
 References
 
1 Blanche S, Newell M-L, Mayaux M-J et al. Morbidity and mortality in European children vertically infected by HIV-1. J Acquir Immune Defic Syndr Hum Retrovirol 1997;14:442–50.[ISI][Medline]

2 Diaz C, Hanson C, Cooper ER et al. Disease progression in a cohort of infants with vertically acquired HIV infection observed from birth: The Women and Infants Transmission Study. J Acquir Immune Defic Syndr Hum Retrovirol 1998;18:221–28.[ISI][Medline]

3 Centers for Disease Control. 1994 revised classification system for human immunodeficiency virus infection in children less than 13 years of age. Morb Mortal Wkly Rep 1994;43(RR-12):1–10.

4 de Martino M, Tovo P-A, Galli L et al. Prognostic significance of immunologic changes in 675 infants perinatally exposed to human immunodeficiency virus. J Pediatr 1991;119:702–09.[ISI][Medline]

5 de Martino M, Tovo P-A, Tozzi AE et al. HIV-1 transmission through breast-milk: appraisal of risk according to duration of feeding. AIDS 1992;6:991–97.[ISI][Medline]

6 Tovo P-A, de Martino M, Gabiano C et al. Prognostic factors and survival in children with perinatal HIV-1 infection. Lancet 1992;339: 1249–53.[ISI][Medline]

7 de Martino M, Tovo P-A, Galli L et al. Features of children perinatally infected with HIV-1 surviving longer than 5 years. Lancet 1994;343: 191–95.[ISI][Medline]

8 Galli L, de Martino M, Tovo P-A et al. Onset of clinical signs in children with HIV-1 perinatal infection. AIDS 1995;9:455–61.[ISI][Medline]

9 Centers for Disease Control and Prevention. 1993 revised classification system for HIV infection and expanded surveillance case definition for AIDS among adolescents and adults. Morb Mortal Wkly Rep 1993;41(RR-17):1–19.

10 Kaplan EL, Meier P. Non parametric estimation from incomplete observations. J Am Stat Assoc 1958;53:457–81.[ISI]

11 Kalbfleisch JD, Pretice RL. The Statistical Analysis of Failure Time Data. Boston: Wiley & Sons, 1980.

12 Diggle PJ. Time Series—A Biostatistical Introduction. New York: Oxford University Press, 1990.

13 Kleimbaum DG, Kupper LL. Applied Regression Analysis and Other Multivariable Methods. Boston: Duxbury Press, 1984.

14 Centers for Disease Control and Prevention. Guidelines for the use of antiretroviral agents in pediatric HIV infection. Morb Mortal Wkly Rep 1998;47(RR-4):1–43.[Medline]

15 Barnhart HX, Calwell MB, Thomas P et al. Natural history of human immunodeficiency virus disease in perinatally infected children: an analysis from the Pediatric Spectrum of Disease Project. Pediatrics 1996;97:710–16.[Abstract]

16 Scarlatti G. Paediatric HIV infection. Lancet 1996;348:863–68.[ISI][Medline]

17 Pliner V, Weedon J, Thomas PA et al. Incubation period of HIV-1 in perinatally infected children. AIDS 1998;12:759–66.[ISI][Medline]

18 Blanche S, Mayaux M-J, Rouzioux C et al. Relation of the course of HIV infection in children to the severity of the disease in their mothers at delivery. N Engl J Med 1994;330:308–12.[Abstract/Free Full Text]

19 Tozzi AE, Pezzotti P, Greco D. Does breast-feeding delay progression to AIDS in HIV-infected children? AIDS 1990;4:1293–94.[ISI][Medline]

20 De Martino M, Tovo P-A, Galli L et al. Rapid disease progression in HIV-1 perinatally infected children born to mothers receiving zidovudine monotherapy during pregnancy. AIDS 1999;13:927–33.[ISI][Medline]

21 Principi N, Marchisio P, Onorato J et al. Long-term administration of aerosolized pentamidine as primary prophylaxis against Pneumocystis carinii pneumonia in infants and children with symptomatic human immunodeficiency virus Infection. J Acquir Immune Defic Syndr Hum Retrovirol 1996;12:158–63.[ISI][Medline]

22 Englund JA, Baker CJ, Raskino C et al. Zidovudine, didanosine, or both as the initial treatment for symptomatic HIV-infected children. N Engl J Med 1997;336:1704–12.[Abstract/Free Full Text]

23 Mocroft A, Johnson MA, Phillips AN. Factors affecting survival in patients with the acquired immunodeficiency syndrome. AIDS 1996; 10:1057–65.[ISI][Medline]

24 Italian Register for HIV Infection in Children. Italian guidelines for antiretroviral therapy in children with human immunodeficiency virus-type 1 infection. Acta Paediatr 1999;88:228–32.[ISI][Medline]