Median time from seroconversion to AIDS in Italian HIV-positive haemophiliacs: different parametric estimates

Estimation of the burden of people living with Human Immunodeficiency Virus/Acquired Immunodeficiency Syndrome (HIV/AIDS) in Kerala state, India

Background

Worldwide, 36.7 million people were infected with Acquired Immunodeficiency Syndrome (AIDS) by the end of 2015. Over the period 2007 to 2015, there was a declining trend in the prevalence of adult Human Immunodeficiency Virus (HIV) in the state of Kerala, India. The current study aims to find a suitable statistical modelling technique for the distribution of HIV incubation time and predict the cumulative number of AIDS cases.

Materials and Methods

The requisite data were obtained from the Kerala State AIDS Control Society (KSACS) for the years 2007 to 2015. To assess the distribution of HIV incubation time, the data of 22 HIV-infected Keralite patients were retrieved from the medical records of a teaching hospital. Data included age, gender, and incubation time. The back-calculation method was utilized to predict the cumulative HIV/AIDS cases.

Results

The estimated total cumulative AIDS cases in Kerala for the years 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014 and 2015 were found to be 35,777, 48,944, 62,039, 45,669, 45,668, and 43,605, 42,377, 39,362, 37,617, 39,583, 25,414 respectively using back-calculation method with Weibull (2) incubation time distribution. The mean incubation time of the total HIV cases (male and female) was 4.4 years which indicates a rapid progression of the disease in the state of Kerala.

Conclusion

The back-calculation method is a powerful tool to estimate the cumulative frequency of AIDS cases; which predicted a declining HIV trend among Keralites. Moreover, the Weibull distribution is the best fitted distribution for HIV incubation time in our population.

[Cohort studies and their contribution to the study of HIV infection: main characteristics and limitations]

In this paper, we provide a definition of cohort studies and reviews the main types of cohort studies used in the context of HIV infection. We discuss how the main sources of selection biases in cohort studies are those derived from the lack of observation of the event that determines the origin and/or of the event of interest due to losses to follow-up or development of a competing event, and how this bias must be appropriately taken into account following specific epidemiological methods. Although cohort studies play an essential role in the study of HIV infection, they are logistically complex and require considerable resources. Therefore, strategic planning on the quality and quantity of the information collected must always be accompanied by a resource allocation plan.

Survival analysis on aggregate data to assess time to sero-conversion after experimental infection with Bovine Leukemia virus

Bovine Leukemia virus (BLV) is a ubiquitous retrovirus that affects mainly cattle. Knowledge of the precise moment of infection is fundamental for identification and evaluation of factors related to BLV transmission. Systematic reviews and meta-analyses provide good evidence on the effects of medical interventions. The objectives were to estimate time to sero-conversion after experimental infection using data from retrieved literature and to detect factors that may influence the length of that interval using survival analysis on pooled data. An analysis using aggregate data from 36 studies totalling 438 observations was performed. From this, four sets were created and analysed by interval-censored accelerated failure time models (AFT) with different distributions (exponential, Weibull, log-logistic, lognormal and generalized gamma), and some variants of the Cox model (Andersen-Gill, smoothing splines) with and without a frailty effect. The AFT gamma model fit best and the estimated median time to sero-conversion in the null model was 57 days (95% confidence interval (CI): 49; 75) using all data and 47 days (95% CI: 39; 55) when only studies using experimental inoculation were considered. Some factors were consistently associated with time to sero-conversion. These included exposure by animal-to-animal contact (resulting in a seven-fold increase in time to sero-conversion compared to direct inoculation), diagnostic method to detect sero-conversion (time to sero-conversion was 1.4 times shorter when AGID was used compared to ELISA), and transmission by insect bites (biological media) delayed sero-conversion 2.3 times compared transmission via needles or other inanimate media. After fitting a frailty Cox model, results showed that sero-conversion in susceptible animals after infection using donors, in which presence of virus before the experiment started was confirmed, increased the hazard of sero-conversion two times in comparison with donors in which virus presence was not confirmed before start of the experiment. Inoculation with blood decreased the hazard 2.5 times in comparison with lymphocyte suspensions. Heterogeneity due to different research groups was also present. Finally, a Cox model with smoothing splines contained three variables: research group, route of inoculation and a non-linear spline for infective dose. In conclusion, it can be stated some factors that influence the time to sero-conversion were identified and quantified and that a moderate influence of research centre existed. These results may contribute to the estimation of the most probable times of infection in field conditions and in a better evaluation of control measures.

Differences in susceptibility to AIDS development: a cohort study of Danish and American homosexual-bisexual men, 1981 to 1995

We estimated the annual hazard rate for progression to AIDS as defined by the 1987 case definition in HIV-infected members of a cohort of Danish and American homosexual-bisexual men who were observed from 1981 to 1995. Furthermore, we extrapolated the hazard to 25 years based on imputed future time to AIDS. Of 201 HIV-positive subjects, 112 developed AIDS before the end of follow-up. The hazard increased rapidly during the first years following infection, attained a peak of about 15% per year at year 7, and was moderately lower during years 8 through 10. In subsequent analysis, we imputed future time to AIDS in 89 subjects who had not progressed by the end of follow-up by extrapolation from subject-specific CD4 trend lines. A CD4 count of < or = 100 cells/microl was the best surrogate for clinical AIDS. Under this model, the imputed AIDS hazard stabilized at around 8% per year after 10 years. We projected that 13% (95% confidence interval [CI], 8%-19%) of the infected men may remain free from AIDS 25 years after seroconversion. Our direct data suggest that incubation times reflect a mixture of a population that is susceptible to disease progression and has short incubation periods with a group that is relatively resistant. Based on an extrapolation model, > 10% of HIV-infected persons may survive for up to 25 years without developing AIDS.

The UK register of HIV seroconverters: methods and analytical issues. UK register of HIV seroconverters (UKRHS) Steering Committee

A Register of HIV-infected persons who have had a negative antibody test within 3 years of their first antibody positive test (seroconverters) is being set up in the UK to monitor the distribution of times from HIV seroconversion to AIDS (the incubation period) and to death. It will also provide a national resource for use by those designing studies in this group of individuals. Clinicians caring for HIV-positive persons in Genito-Urinary Medicine, Infectious Disease and other departments throughout the UK were asked to participate by providing information on eligible subjects. Most laboratories undertaking HIV antibody testing were also contacted and asked to provide the name of the attending clinician for all seroconverters identified through the HIV laboratory reporting systems of the PHLS Communicable Disease Surveillance Centre (CDSC) and the Scottish Centre for Infection and Environmental Health (SCIEH) and for any other seroconverters known to them but not identified by CDSC or SCIEH. Data items sought for the Register include: sex, ethnic group, probable route of HIV transmission, annual CD4 counts, details of therapy and prophylaxis prescribed, AIDS-defining events and vital status. Follow up information is collected annually. Wherever possible, all seroconverters known to a clinic have been identified, whether currently alive or dead, either from clinic records or laboratory reporting or both. The objective is to establish and update a complete register of seroconverters on a long-term to basis to provide reliable estimates of the incubation period on which future projections of AIDS cases in the UK can be made.

Analysis of an HIV-infected cohort followed for as long as 15 years after seroconversion

Data from a cohort of 62 HIV-positive individuals with hemophilia or von Willebrands disease infected for a maximum period of 15 years were analyzed. The relation between CD4+ and total lymphocyte counts and their rate of decline was analyzed with respect to age at seroconversion, time of seroconversion, and development of disease and subsequent death. As expected, the CD4+ and total lymphocyte population decline correlated with increased probability of disease and death. The patients fell into two distinct categories with respect to this decline: those whose cell count declined steadily (single slope) and those whose cell count remained steady or decreased very slowly for a variable period and then declined sharply (double slope). Within this cohort, the presence of a double slope appears to indicate a poorer prognosis, as 9 of 18 of the patients who have died showed this pattern, whereas only 6 of 42 of the remaining patients have this pattern even though more than half of this group have CD4+ lymphocyte counts < 0.2 x 10(9)/L. In addition, the ratio of CD4+ lymphocyte count to total lymphocyte count decreased with increasing cumulative frequency of the cumulative incidence of disease and death and the overall probability of death in this cohort was lower than expected, being 30% 12 years after seroconversion.

A meta-analysis of estimates of the AIDS incubation distribution

Information from 12 studies is combined to estimate the AIDS incubation distribution with greater precision than is possible from a single study. The analysis uses a hierarchy of parametric models based on a four-parameter generalized F distribution. This general model contains four standard two-parameter distributions as special cases. The cases are the Weibull, gamma, log-logistic, lognormal distributions. These four special cases subsume three distinct asymptotic hazard behaviors. As time increases beyond the median of approximately 10 years, the hazard can increase to infinity (Weibull), can plateau at some constant level (gamma), or can decrease to zero (log-logistic and lognormal). The Weibull, gamma and 'log-logistic distributions' which represent the three distinct asymptotic hazard behaviors, all fit the data as well as the generalized F distribution at the 25 percent significance level. Hence, we conclude that incubation data is still too limited to ascertain the specific hazard assumption that should be utilized in studies of the AIDS epidemic. Accordingly, efforts to model the AIDS epidemic (e.g., back-calculation approaches) should allow the incubation distribution to take several forms to adequately represent HIV estimation uncertainty. It is recommended that, at a minimum, the specific Weibull, gamma and log-logistic distributions estimated in this meta-analysis should all be used in modeling the AIDS epidemic, to reflect this uncertainty.

Epidemiology of hemophilia and of HIV infection in Italy. GICC. Gruppo Italiano Coagulopatie Congenite

To evaluate the incidence and prevalence of hemophilia in Italy and the impact of HIV infection on the Italian hemophiliac population, data from a computerized national registry of patients from 95% of the hemophilia care centers in Italy were analyzed. A total of 4643 patients were included in the registry. The prevalence of hemophilia A was 8.2 per 100,000 males, with no significant regional differences; for hemophilia B the corresponding figure was 1.5 per 100,000. Temporal trends in hemophilia incidence suggest that the diagnosis of mild and moderate hemophilia has improved. The overall HIV prevalence was 26% and was significantly (p < 0.001) higher in patients with hemophilia B (47.1%) compared to those with hemophilia A (26.8%) or other diseases (16.5%). The highest rate of HIV seropositivity was among patients 20-29 years of age. The annual amount of clotting factor concentrates received was significantly (p < 0.001) higher in HIV seropositive patients than in those who were seronegative. Antibody testing was never performed on 10.1% of severely affected patients. The number of patients in the Italian registry was similar to the number that would have been expected based on prevalence estimates from other countries. In comparison with other countries, the prevalence of HIV infection recorded in Italy was lower in persons with hemophilia A, but higher in those with hemophilia B. Our study demonstrates the usefulness of a registry in delineating the epidemiology of hemophilia and in studying risk factors for HIV infection. It also underlines the need for continuing surveillance of this population.

A model for AIDS pathogenesis

The idea is put forward and analysed numerically, that within an infected person HIV evolves to increase its reproductivity within the population of CD4+ cells. A mathematical model predicts initial viremia and CD4+ cell drop after HIV infection and thereafter a slow progressive decline in the number of CD4+ cells, although for an extended period HIV is kept at a relatively low level by an active immune response. The time span T until the number of CD4+ cells falls below 20 per cent of its normal value depends on several model parameters. Assuming Gaussian distributions for these parameters, the model predicts a distribution function for T which resembles the observed distribution function for the incubation period to AIDS.