Estimating incidence from prevalence in generalised HIV epidemics: methods and validation.

BACKGROUND: HIV surveillance of generalised epidemics in Africa primarily relies on prevalence at antenatal clinics, but estimates of incidence in the general population would be more useful. Repeated cross-sectional measures of HIV prevalence are now becoming available for general populations in many countries, and we aim to develop and validate methods that use these data to estimate HIV incidence. METHODS AND FINDINGS: Two methods were developed that decompose observed changes in prevalence between two serosurveys into the contributions of new infections and mortality. Method 1 uses cohort mortality rates, and method 2 uses information on survival after infection. The performance of these two methods was assessed using simulated data from a mathematical model and actual data from three community-based cohort studies in Africa. Comparison with simulated data indicated that these methods can accurately estimates incidence rates and changes in incidence in a variety of epidemic conditions. Method 1 is simple to implement but relies on locally appropriate mortality data, whilst method 2 can make use of the same survival distribution in a wide range of scenarios. The estimates from both methods are within the 95% confidence intervals of almost all actual measurements of HIV incidence in adults and young people, and the patterns of incidence over age are correctly captured. CONCLUSIONS: It is possible to estimate incidence from cross-sectional prevalence data with sufficient accuracy to monitor the HIV epidemic. Although these methods will theoretically work in any context, we have able to test them only in southern and eastern Africa, where HIV epidemics are mature and generalised. The choice of method will depend on the local availability of HIV mortality data.

Survival after HIV infection in the pre-antiretroviral therapy era in a rural Tanzanian cohort.

BACKGROUND: Survival patterns after HIV infection in African populations in the era before antiretroviral therapy (ART) form an important baseline for measuring future successes of treatment programmes. Few studies have followed seroconverters for 10 or more years to describe such patterns. METHODS: The Kisesa open cohort study conducted four rounds of village-based HIV testing and 20 rounds of household-based demographic surveillance between 1994 and 2006. Approximate infection dates were established for individual seroconverters by allocating a date between the last negative and first positive test. Person-years lived post-infection were computed, allowing for left truncation and right censoring, and Kaplan-Meier survival functions were constructed, truncating the analysis at the start of 2005 when ART first became available in the community. Weibull models were fitted to estimate median survival time, and parametric regression methods were used to investigate the influence of sex and age at infection. RESULTS: A total of 369 seroconverters were identified, providing 890 person-years of follow-up during which 44 deaths were observed. The Kaplan-Meier function showed 67% surviving 9 years post-infection, and the overall predicted median survival was 11.5 years. Survival was strongly related to age at infection (hazard ratio 1.06 for each additional year of age, and weakly to sex. A strong effect of age was evident even after allowing for mortality from non-HIV-related causes using cause deletion methods to estimate net mortality. CONCLUSION: The survival of HIV-infected individuals was comparable to that reported in developed country studies before the introduction of HAART. Survival patterns in Kisesa are marginally more favourable than those reported in cohort studies in Uganda.

Time from HIV seroconversion to death: a collaborative analysis of eight studies in six low and middle-income countries before highly active antiretroviral therapy.

OBJECTIVES: To estimate survival patterns after HIV infection in adults in low and middle-income countries. DESIGN: An analysis of pooled data from eight different studies in six countries. METHODS: HIV seroconverters were included from eight studies (three population-based, two occupational, and three clinic cohorts) if they were at least 15 years of age, and had no more than 4 years between the last HIV-negative and subsequent HIV-positive test. Four strata were defined: East African cohorts; South African miners cohort; Thai cohorts; Haitian clinic cohort. Kaplan-Meier functions were used to estimate survival patterns, and Weibull distributions were used to model and extend survival estimates. Analyses examined the effect of site, age, and sex on survival. RESULTS: From 3823 eligible seroconverters, 1079 deaths were observed in 19 671 person-years of follow-up. Survival times varied by age and by study site. Adjusting to age 25-29 years at seroconversion, the median survival was longer in South African miners: 11.6 years [95% confidence interval (CI) 9.8-13.7] and East African cohorts: 11.1 years (95% CI 8.7-14.2) than in Haiti: 8.3 years (95% CI 3.2-21.4) and Thailand: 7.5 years (95% CI 5.4-10.4). Survival was similar for men and women, after adjustment for age at seroconversion and site. CONCLUSION: Without antiretroviral therapy, overall survival after HIV infection in African cohorts was similar to survival in high-income countries, with a similar pattern of faster progression at older ages at seroconversion. Survival appears to be significantly worse in Thailand where other, unmeasured factors may affect progression.

Comparison of HIV prevalences in community-based and antenatal clinic surveys in rural Mwanza, Tanzania.

OBJECTIVES: First, to compare the prevalence of HIV infection among women in the general population and antenatal clinic (ANC) attenders in rural Mwanza, Tanzania, and second, to validate a method for adjusting HIV prevalence in ANC attenders to estimate the prevalence in the general female population aged 15-44 years. METHODS: A cross-sectional population survey was conducted in 12 rural communities of Mwanza Region between 1991 and 1992. From the same communities sequential ANC attenders were recruited on two occasions between 1991 and 1993. Consenting subjects were interviewed, examined, treated and a serum sample was tested for HIV. The HIV prevalence in women in the general population was compared with unadjusted and adjusted prevalences in ANC attenders. Parity-adjusted prevalences were obtained by applying correction factors to the observed prevalences in parous and nulliparous ANC attenders. RESULTS: A total of 5675 women aged 15-44 years from the general population and 2265 ANC attenders had complete socio-demographic and laboratory data. Unadjusted HIV prevalence was significantly lower in ANC attenders (3.6%) than women from the general population (4.7%, P = 0.025), but after adjustment there was no significant difference between the two groups (4.6 versus 4.7%, P = 0.95). CONCLUSION: In this rural population, the HIV prevalence in ANC attenders underestimated the prevalence among women in the general population, but this difference was eliminated by applying parity-based correction factors. Information on parity should be routinely collected in ANC-based HIV sentinel surveillance.