- Research article
- Open Access
- Open Peer Review
AIDS-defining illnesses among patients with HIV in Singapore, 1985 to 2001: results from the Singapore HIV Observational Cohort Study (SHOCS)
BMC Infectious Diseasesvolume 4, Article number: 47 (2004)
The objective was to describe the causes of initial and overall AIDS-defining disease episodes among HIV patients in Singapore.
A retrospective observational cohort study was performed of all adult patients seen at the national HIV referral center between 1985 and 2001. Data were extracted from the patients' records by ten trained healthcare workers. AIDS-defining conditions were established using predefined criteria.
Among 1504 patients, 834 had experienced one or more AIDS-defining diseases. The most frequent causes of the initial AIDS-defining episode were Pneumocystis carinii pneumonia (35.7%), Mycobacterium tuberculosis (22.7%) and herpes simplex (7.4%). In total 1742 AIDS-defining episodes occurred. The most frequent causes were Pneumocystis carinii pneumonia (25.1%), Mycobacterium tuberculosis (16.2%) and cytomegalovirus retinitis (9.5%).
The most frequent causes of AIDS-defining illnesses in Singapore are similar to those reported in the West, prior to the introduction of anti-retroviral therapy. Opportunistic infections remain the most frequent AIDS-defining illnesses.
Cohort studies have provided valuable information on the clinical course of HIV infection in patients from Europe [1–16], North America [3, 17–21], South America [22, 23] and Africa [24–32]. The introduction of antiretroviral therapy has dramatically altered the incidence of AIDS-defining illnesses in the West. In the EuroSIDA study there have been substantial falls in the incidence and total number of AIDS-defining episodes between 1994 and 1998 . During this period the proportions of reported AIDS-defining illnesses due to cytomegalovirus retinitis and Mycobacterium avium have decreased from 9% to 2% and 8% to 3% respectively . The proportion due to non-Hodgkin's lymphoma has increased from 4% to 16% . Substantial decreases in the incidence of disease caused by cytomegalovirus, Pneumocystis carinii, Mycobacterium avium and other opportunistic infections have also been observed in the United States [20, 21].
Relatively few data are available on the course of HIV infection in Asian populations. In Bangkok between 1987 and 1993 the most frequent AIDS-defining diagnoses were extrapulmonary tuberculosis (22.8%), Pneumocystis carinii pneumonia (7.0%) and cryptococcal meningitis (10.9%) . Little is known regarding whether the pattern of HIV-related conditions has changed following the introduction of antiretroviral therapy in Asia.
The Communicable Diseases Centre (CDC) in Singapore is the national reference centre for adult patients with HIV and AIDS and nationwide 94.9% of all Singaporean adult residents, who have ever been diagnosed, have been referred there. Underreporting of HIV does not occur as reporting occurs automatically from the reference laboratory. Records have been kept since the first case was identified in Singapore in 1985. Therefore the Singapore HIV Observational Cohort Study (SHOCS) contains almost the entire country's HIV experience. Nucleoside analogues have been available in Singapore since the early 1990s and protease inhibitors and non-nucleoside reverse transcriptase inhibitors since 1997. The usage of these drugs has been increasing annually and during 2000 and 2001 approximately 70–80% of patients regularly attending CDC were taking some form of anti-retroviral therapy.
The details of the SHOCS cohort have been described previously . The study was approved by the Ethics Committee of Tan Tock Seng Hospital. Data were extracted from the case notes of all Singaporean residents who were seen at CDC on or before 31st December 2001, by ten trained healthcare workers. Probable and confirmed criteria were developed for the diagnosis of category C  and category B (AIDS-related complex) conditions, based on the 1993 guidelines of the United States' Centers for Disease Control and Prevention .
All disease episodes were initially determined by the data extractors and then checked by an infectious disease physician (RB). Diagnoses were not included if they did not fulfill the specified criteria even if there was a strong clinical suspicion of a particular condition. If there was insufficient evidence to satisfy the criteria for a specific diagnosis, a more general diagnosis was assigned, for example cerebral lesion (cause unknown) would be used if there was insufficient evidence to support a diagnosis of toxoplasmosis of the brain or primary cerebral lymphoma.
Median CD4 counts were calculated based on the sample taken nearest the time of diagnosis of the initial AIDS-defining condition. CD4 counts from 893 patients were used, as in 37 (4.0%) cases no suitable count was available. Data were entered into a computer database (Microsoft Access™) and checked for errors and inconsistencies. Statistical analyses were performed using Stata 7.0™. Each new or recurrent AIDS-defining condition counted as one event (i.e. a patient with three diagnoses would contribute three events). An AIDS-defining condition was counted as a separate additional event if it recurred six or more months after the initial diagnosis or in the case of tuberculosis, six or more months after treatment completion. To examine the effects of highly active anti-retroviral therapy (HAART) on the proportions of AIDS-defining illnesses due to each of the three most common infections (Pneumocystis carinii pneumonia, Mycobacterium tuberculosis and cytomegalovirus retinitis), comparisons were made between the pre-HAART era (1986 to 1995) and the established HAART era (2000 to 2001) using χ2 tests.
Among the 1504 patients infected with HIV who were seen at CDC between 1985 and the end of 2001, 834 developed one or more AIDS-defining conditions. The most frequent initial AIDS-defining disease episodes were due to Pneumocystis carinii pneumonia (35.7% of total diagnoses), M. tuberculosis (22.7%), herpes simplex (chronic mucocutaneous) (7.4%) and candidiasis (esophageal or tracheobronchial) (6.9%) (table 1). The median CD4 count at which the initial AIDS-defining condition occurred was 27 (inter-quartile range 11 – 63). The median CD4 count was higher for M. tuberculosis infection (52, IQR = 18 – 110) and Kaposi's sarcoma (46.5, IQR = 23.5 – 227.5) than for Pneumocystis carinii pneumonia (16.5, IQR = 9 – 40), herpes simplex (25, IQR = 9 – 63) and extrapulmonary cryptococcosis (13, IQR = 6.5 – 39) (table 1).
1742 AIDS-defining disease episodes were recorded. The 10 most frequent AIDS-defining conditions were all of infectious etiology. There were 1658 first episodes of disease and 84 recurrences. The most frequent AIDS-defining diseases were Pneumocystis carinii pneumonia (25.1% of total diagnoses), M. tuberculosis (16.2%), cytomegalovirus retinitis (9.5%), candidiasis (7.8%), chronic mucocutaneous herpes simplex (8.2%) and disseminated M. avium (8.2%) (table 2). Several additional conditions could have included undiagnosed AIDS-defining diseases. The most common of these infections was pneumonia of unknown cause (117 cases), which is likely to include some patients with Pneumocystis carinii pneumonia (table 3). Weight loss of >10% body weight was a frequent problem and affected 383 (25.5%) patients (table 3).
Between 1996 and 2001 there was a continuous increase in the total number of AIDS-defining disease episodes occurring each year, due to the increase in the number of patients being followed up. There was no significant change in the overall proportion of AIDS-defining episodes caused by infections. Pneumocystis carinii and M. tuberculosis remained the two commonest causes. Between 1986 and 1995 they respectively accounted for 26.7% and 10.8% of all AIDS-defining disease episodes. During 2000 to 2001 the percentage of episodes caused by Pneumocystis carinii was not significantly different at 26.3% (Yates' χ2 = 0.01, 2df, P = 0.94), but that due to M. tuberculosis had increased to 18.0% (Yates' χ2 = 8.10, 2df, P = 0.004). There was no significant change in the percentage of episodes due to cytomegalovirus retinitis (9.7% between 1986 and 1995 and 8.5% during 2000 and 2001; Yates' χ2 = 0.25, 2df, P = 0.62) (table 4).
Comparison of HIV-associated morbidity with other populations
In the SHOCS cohort the proportion of conditions which were of infectious etiology remained very high throughout the study period. The distribution of conditions was similar to that seen in Western countries prior to the introduction of anti-retroviral therapy . However the proportion of conditions caused by mycobacteria was higher than in Europe [1, 12]. In Bangkok the most common AIDS-defining conditions were infections. Between 1986 and 1993 the most frequent AIDS-defining illnesses in Bangkok were extra-pulmonary tuberculosis (22.8%), cryptococcal meningitis (10.9%) and Pneumocystis carinii pneumonia (7%) . Between 1993 and 1996 they were extrapulmonary cryptococcosis (38.4%), tuberculosis (37.4%), wasting syndrome (8.1%) and Pneumocystis carinii pneumonia (4.8%) . Pneumocystis carinii pneumonia caused a much higher proportion of AIDS-defining diseases in Singapore than in Bangkok and cryptococcal meningitis occurred less frequently in Singapore.
Potential sources of bias
The SHOCS cohort provides data on the experience of 94.9% of Singaporean residents who have been diagnosed with HIV since the epidemic began. There are good diagnostic facilities in Singapore and patients with HIV-related illness receive extensive investigations. Therefore Singapore offers an ideal location for collecting information on the causes of morbidity and mortality in persons with HIV in Asia. Medical record keeping is of a high standard in our hospital and this allowed accurate diagnoses to be assigned for AIDS-defining conditions which occurred throughout the AIDS era. To ensure consistency, data was extracted by carefully trained healthcare workers and diagnoses were assigned using predefined criteria. All AIDS-defining diagnoses and all causes of death were reviewed by the same infectious diseases physician (RB). Follow-up rates in our cohort are very high with only 5.8% of patients lost to follow-up at 12 months and 9.3% after 3 years. The SHOCS cohort therefore provides a unique source of information on HIV infection in an Asian country since the start of the epidemic.
In a retrospective study it is impossible to ensure that all patients received a full series of investigations in order to satisfy the study criteria for diagnosis of HIV-related conditions. As a consequence the criteria for making a probable diagnosis are unavoidably biased against certain conditions. For example if a patient undergoes a therapeutic trial of tuberculosis treatment he or she can be given the diagnosis of "probable Mycobacterium tuberculosis". However if a patient receives a successful trial of treatment for disseminated M. avium, the diagnosis is "probable mycobacteriosis (species unidentified)".
Differences in physicians' awareness of the link between HIV and opportunistic infections can influence diagnosis rates. For example, the increase in the proportion of AIDS-defining disease episodes caused by tuberculosis between 1986–1995 and 2000–2001 may have been due to greater awareness of the link between HIV and tuberculosis. Diagnosis rates for M. avium and M. tuberculosis may also have been affected by improvements in mycobacterial culture techniques.
Despite the availability of anti-retroviral medication, opportunistic infections remain a common problem among HIV patients in Singapore. This may partly be explained by the high proportion of Singaporean patients, who present with advanced disease and very low CD4 counts. These patients often already have one or more AIDS-defining opportunistic infections at first presentation. Opportunistic infections may also occur before adequate immune-reconstitution can occur when HAART is commenced when the patient has a very low CD4 count. Studies in the West have shown that AIDS-defining illnesses occur more frequently among patients who have a CD4 count below 50 when anti-retroviral therapy is commenced than among those with higher CD4 counts [37, 38]. Immune-reconstitution may also be impaired by difficulties in maintaining high levels of adherence, sub-optimal treatment regimens and unplanned treatment interruptions (due to adverse effects, financial or logistical reasons).
The current reductions in the cost of anti-retroviral therapy (Eg. due to generic manufacture) may enable high levels of anti-retroviral use to be achieved in many Asian countries. The experiences of the SHOCS cohort are likely to be repeated throughout Asia. Our results suggest that avoidable opportunistic infections may continue to occur even when high levels of anti-retroviral use are achieved. To reduce the burden of morbidity and mortality caused by these infections, efforts must be made to diagnose HIV infection earlier and to make the treatment of HIV and its associated opportunistic infections more affordable.
= Singapore HIV observational cohort study
= Communicable Diseases Centre
= acquired immunodeficiency syndrome
= human immunodeficiency virus
= highly active anti-retroviral therapy
Pezzotti P, Serraino D, Rezza G, Dal Maso L, Vaccher E, Lepri AC, Franceschi S: The spectrum of AIDS-defining diseases: temporal trends in Italy prior to the use of highly active anti-retroviral therapies, 1982–1996. Int J Epidemiol. 1999, 28: 975-981. 10.1093/ije/28.5.975.
Lundgren JD, Phillips AN, Pedersen C, Clumeck N, Gatell JM, Johnson AM, Ledergerber B, Vella S, Nielsen JO, for the AIDS in Europe Study Group: Comparison of long-term prognosis of patients with AIDS treated and not treated with zidovudine. JAMA. 1994, 271: 1088-1092. 10.1001/jama.271.14.1088.
Veugelers PJ, Page KA, Tindall B, Schechter MT, Moss AR, Winkelstein WW, Cooper DA, Craib KJP, Charlebois E, Coutinho RA, van Griensven GJP: Determinants of HIV disease progression among homosexual men registered in the Tricontinental Seroconverter Study. Am J Epidemiol. 1994, 140: 747-758.
Egger M, Hirschel B, Francioli P, Sudre P, Wirz M, Flepp M, Rickenbach M, Malinverni R, Vernazza P, Battegay M, and the Swiss HIV Cohort Study: Impact of new antiretroviral combination therapies in HIV infected patients in Switzerland: prospective multicentre study. BMJ. 1997, 315: 1194-1199.
Phillips AN, Katlama C, Barton S, Vella S, Blaxhult A, Clotet B, Goebel F-D, Hirschel B, Pedersen C, Lundgren JD, for the EUROSIDA Study Group: Survival in 2367 zidovudine-treated patients according to use of other nucleoside analogue drugs. J Acquir Immune Defic Syndr Hum Retrovirol. 1998, 17: 239-244.
Mocroft A, Vella S, Benfield TL, Chiesi A, Miller V, Gargalianos P, d'Arminio Monforte A, Yust I, Bruun JN, Phillips AN, Lundgren JD, for the EUROSIDA Study Group: Changing patterns of mortality across Europe in patients with HIV-1. Lancet. 1998, 352: 1725-1730. 10.1016/S0140-6736(98)03201-2.
Miller V, Mocroft A, Reiss P, Katlama C, Papadopoulos AI, Katzenstein T, van Lunzen J, Antunes F, Phillips AN, Lundgren JD, for the EUROSIDA study group: Relations among CD4 lymphocyte count nadir, antiretroviral therapy, and HIV-1 disease progression: results from the EuroSIDA study. Ann Intern Med. 1999, 130: 570-577.
Mocroft A, Kirk O, Barton SE, Dietrich M, Proenca R, Colebunders R, Pradier C, d'Arminio Monforte A, Ledergerber B, Lundgren JD, for the EUROSIDA Study Group: Anaemia is an independent predictive marker for clinical prognosis in HIV-infected patients from across Europe. AIDS. 1999, 13: 943-950. 10.1097/00002030-199905280-00010.
Chiesi A, Mocroft A, Dally LG, Miller V, Katlama C, Ledergerber B, Pedersen C, Phillips AN, Arcieri R, Lundgren JD, for the EUROSIDA Study Group: Regional survival differences across Europe in HIV-positive people: the EuroSIDA study. AIDS. 1999, 13: 2281-2288. 10.1097/00002030-199911120-00010.
Mocroft A, Katlama C, Johnson AM, Pradier C, Antunes F, Mulcahy F, Chiesi A, Phillips AN, Kirk O, Lundgren JD, for the EUROSIDA Study Group: AIDS across Europe, 1994–1998: the EuroSIDA study. Lancet. 2000, 356: 291-296. 10.1016/S0140-6736(00)02504-6.
Paredes R, Mocroft A, Kirk O, Lazzarin A, Barton SE, van Lunzen J, Katzenstein TL, Antunes F, Lundgren JD, Clotet B, for the EUROSIDA Study Group: Predictors of virological success and ensuing failure in HIV-positive patients starting highly active antiretroviral therapy in Europe. Results from the EuroSIDA study. Arch Intern Med. 2000, 160: 1123-1132. 10.1001/archinte.160.8.1123.
Kirk O, Gatell JM, Mocroft A, Pedersen C, Proenca R, Brettle RP, Barton SE, Sudre P, Phillips AN, Lundgren JD, for the EUROSIDA Study Group: Infections with Mycobacterium tuberculosis and Mycobacterium avium among HIV-infected patients after the introduction of highly active antiretroviral therapy. Am J Respir Crit Care Med. 2000, 162: 865-872.
Viard J-P, Mocroft A, Chiesi A, Kirk O, Roge B, Panos G, Vetter N, Bruun JN, Johnson M, Lundgren JD, for the EUROSIDA Study Group: Influence of age on CD4 cell recovery in human immunodeficiency-infected patients receiving highly active antiretroviral therapy: evidence from the EuroSIDA study. J Infect Dis. 2001, 183: 1290-1294. 10.1086/319678.
Mocroft A, Phillips AN, Miller V, Gatell J, van Lunzen J, Parkin JM, Weber R, Roge B, Lazzarin A, Lundgren JD, on behalf of the EUROSIDA Study Group: The use of and response to second-line protease inhibitor regimens: results from the EuroSIDA study. AIDS. 2001, 15: 201-209. 10.1097/00002030-200101260-00009.
Lundgren JD, Mocroft A, Gatell JM, Ledergerber B, D'Arminio Monforte A, Hermans P, Goebel F-D, Blaxnult A, Kirk O, Phillips AN, for the EUROSIDA Study Group: A clinically prognostic scoring system for patients receiving highly active antiretroviral therapy: results from the EuroSIDA study. J Infect Dis. 2002, 185: 178-187. 10.1086/338267.
Mocroft A, Brettle R, Kirk O, Blaxhult A, Parkin JM, Antunes F, Francioli P, d'Arminio Monforte A, Fox Z, Lundgren JD, for the EUROSIDA Study Group: Changes in the cause of death among HIV positive subjects across Europe: results from the EuroSIDA study. AIDS. 2002, 16: 1663-1671. 10.1097/00002030-200208160-00012.
Montaner JSG, Le TN, Le N, Craib KJP, Schechter MT: Application of the World Health Organization system for HIV infection in a cohort of homosexual men in developing a prognostically meaningful staging system. AIDS. 1992, 6: 719-724.
Stein DS, Graham NMH, Park LP, Hoover DR, Phair JP, Detels R, Ho M, Saah AJ, for the Multicenter AIDS Cohort Study: The effect of the interaction of acyclovir with zidovudine on progression to AIDS and survival. Analysis of data in the Multicenter AIDS Cohort Study. Ann Intern Med. 1994, 121: 100-108.
Gallant JE, Moore RD, Keruly J, Richman DD, Chaisson RE, and the Zidovudine Epidemiology Study Group: Lack of association between acyclovir use and survival in patients with advanced human immunodeficiency virus disease treated with zidovudine. J Infect Dis. 1995, 172: 346-352.
Palella FJ, Delaney KM, Moorman AC, Loveless MO, Fuhrer J, Satten GA, Aschman DJ, Holmberg SD, and the HIV Outpatient Study Investigators: Declining morbidity and mortality among patients with advanced human immunodeficiency virus infection. N Engl J Med. 1998, 338: 853-860. 10.1056/NEJM199803263381301.
Hogg RS, Heath KV, Yip B, Craib KJP, O'Shaughnessy MVO, Schechter MT, Montaner JSG: Improved survival among HIV-infected individuals following initiation of antiretroviral therapy. JAMA. 1998, 279: 450-454. 10.1001/jama.279.6.450.
Chequer P, Hearst N, Hughes ES, Castilho E, Rutherford G, Loures L, Rodrigues L, and the Brazilian State AIDS Program Coordinators: Determinants of survival in adult Brazilian AIDS patients, 1982–1989. AIDS. 1992, 6: 483-487.
Fonseca LAM, Reingold AL, Casseb JR, Brigido LFM, Duarte AJS: AIDS incidence and survival in a hospital-based cohort of asymptomatic HIV seropositive patients in Sao Paolo, Brazil. Int J Epidemiol. 1999, 28: 1156-1160. 10.1093/ije/28.6.1156.
Lifson AR, Allen S, Wolf , Serufilira A, Kantarama G, Lindan CP, Hudes ES, Nsengumuremyi F, Taelman H, Batnugwanayo J: Classification of HIV infection and disease in women from Rwanda. Evaluation of the World Health Organization staging system and recommended modifications. Ann Intern Med. 1995, 122: 262-270.
Anzala OA, Nico JD, Nagelkerke NJD, Bwayo JJ, Holton D, Moses S, Ngugi EN, Ndinya-Achola JO, Plummer FA: Rapid progression to disease in African sex workers with human immunodeficiency virus type 1 infection. J Infect Dis. 1995, 171: 686-689.
Morgan D, Maude GH, Malamba SS, Okongo MJ, Wagner H-U, Mulder DW, Whitworth JA: HIV-1 disease progression and AIDS-defining disorders in rural Uganda. Lancet. 1997, 350: 245-250. 10.1016/S0140-6736(97)01474-8.
Morgan D, Malamba SS, Maude GH, Okongo MJ, Wagner H-U, Mulder DW, Whitworth JA: An HIV-1 natural history cohort and survival times in rural Uganda. AIDS. 1997, 11: 633-640. 10.1097/00002030-199705000-00011.
Okongo M, Morgan D, Mayanja B, Ross A, Whitworth J: Causes of death in a rural, population-based human immunodeficiency virus type 1 (HIV-1) natural history cohort in Uganda. Int J Epidemiol. 1998, 27: 698-702. 10.1093/ije/27.4.698.
Malamba SS, Morgan D, Clayton T, Mayanja B, Okongo M, Whitworth J: The prognostic value of the World Health Organization staging system for HIV infection and disease in rural Uganda. AIDS. 1999, 13: 2555-2562. 10.1097/00002030-199912240-00009.
Sewenkambo NK, Gray RH, Ahmad S, Serwadda D, Wabwire-Mangen F, Nalugoda F, Kiwanuka N, Lutalo T, Kigozi G, Chuanjun L, Meehan MP, Brahmbatt H, Wawer MJ: Mortality associated with HIV infection in rural Rakai District, Uganda. AIDS. 2000, 14: 2391-2400. 10.1097/00002030-200010200-00021.
Post FA, Badri M, Wood R, Maartens G: AIDS in Africa-survival according to AIDS-defining illness. S Afr Med J. 2001, 91: 583-586.
Morgan D, Mahe C, Mayanja B, Whitworth JAG: Progression to symptomatic disease in people infected with HIV-1 in rural Uganda: prospective cohort study. BMJ. 2002, 324: 193-197. 10.1136/bmj.324.7331.193.
Kitayaporn D, Tansuphaswadikul S, Lohsomboon P, Pannachet K, Kaewkungwal J, Limpakarnjanarat K, Mastro TD: Survival of AIDS patients in the emerging epidemic in Bangkok, Thailand. J Acquir Immune Defic Syndr Hum Retrovirol. 1996, 11: 77-82.
Bellamy R, Sangeetha S, Paton NI: Causes of death among patients with HIV in Singapore, from 1985 to 2001: results from the Singapore HIV Observational Cohort Study (SHOCS). HIV Medicine. 2004, 5: 289-295. 10.1111/j.1468-1293.2004.00223.x.
[No authors listed]: 1993 Revised Classification system for HIV infection and expanded surveillance case definition for AIDS among adolescents and adults. M M W R Recomm Rep. 1992, 41 (RR-17): 1-19.
Tansuphasawadikul S, Amornkul PN, Tanchanpong C, Limpakarnjanarat K, Kaewkungwal J, Likanonsakul S, Eampokalap B, Naiwatanakul T, Kitayaporn D, Young NL, Hu DJ, Mastro TD: Clinical presentation of hospitalized adult patients with HIV infection and AIDS in Bangkok, Thailand. J Acquir Immune Defic Syndr. 1999, 21: 326-332.
Egger M, May M, Chene G, Phillips AN, Ledergerber B, Dabis F, Costagliola D, d'Arminio Monforte A, de Wolf F, Reiss P, Lundgren JD, Justice AC, Staszewski S, Leport C, Hogg RS, Sabin CA, Gill MJ, Salzberger B, Sterne JAC, and the ART Cohort Collaboration: Prognosis of HIV-1-infected patients starting highly active antiretroviral therapy: a collaborative analysis of prospective studies. Lancet. 2002, 360: 119-129. 10.1016/S0140-6736(02)09411-4.
Miller V, Phillips AN, Clotet B, Mocroft A, Ledergerber B, Kirk O, Ormaasen V, Gargalianos-Kakolyris P, Vella S, Lundgren JD, for the EUROSIDA Study Group: Association of virus load, CD4 cell count, and treatment with clinical progression in human immunodeficiency virus-infected patients with very low CD4 cell counts. J Infect Dis. 2002, 186: 189-197. 10.1086/341466.
The pre-publication history for this paper can be accessed here:http://www.biomedcentral.com/1471-2334/4/47/prepub
The Singapore HIV Observational Cohort Study was funded by a grant from the National Healthcare Group, Singapore. We are grateful to Ms Marline Yap, Dr Rumiana Plugarova, Mrs Monica Kalra, Dr Ambihai Sivalingam, Dr Vidhya Gupta, Mrs Prithi Minhas, Dr Celin Chacko, Dr Mona Lisa Rodrigues, Dr Samon Husna and Mrs Manisha Kishore for their meticulous extraction of information from the patients' records. We also gratefully acknowledge the excellent administrative support provided by Dr Bernard Peperstraete, Ms Ravathi Subramaniam and the staff of the Infectious Diseases Research Centre.
The authors declare that they have no competing interests.
RB participated in the design of the study, supervised data extraction, performed the statistical analysis and wrote the manuscript. SS designed the database and supervised data extraction. NP participated in the design of the study and contributed to the statistical analysis and production of the manuscript. All authors read and approved the final manuscript.