Studying dynamics of the HIV epidemic: population-based data compared with sentinel surveillance in Zambia

Abstract

Objectives: To establish population-based HIV survey data in selected populations, and to assess the validity of extrapolation from HIV sentinel surveillance amongst antenatal clinic attenders (ANC) to the general population.

Methods: In a population survey, adults aged > or = 15 years were selected by stratified random cluster sampling (n = 4195). The survey was carried out in catchment populations of clinics used for national HIV surveillance. The methodology allows detailed comparisons of HIV infection patterns to be made in two areas (urban and rural). Whereas the sentinel surveillance used serum-based HIV testing, the population survey used saliva (93.5% consented to provide a saliva sample).

Results: Surveillance of ANC tended to underestimate the overall HIV prevalence of the general population, but differences were not statistically significant. In the urban area, the adjusted overall HIV prevalence rate of ANC (aged 15-39 years) was 24.4% [95% confidence interval (CI), 20.9-28.0] compared with 26.0% (95% CI, 23.4-28.6) in the general population. The respective rural estimates were 12.5% (95% CI, 9.3-15.6) versus 16.4% (95% CI, 12.1-20.6). Age-specific prevalence rates showed ANC to overestimate infection in teenagers (aged 15-19 years), whereas in the reverse direction of those aged > or = 30 years. Teenagers analysed by single year of age revealed both ANC and women in the general population with about the same steep increase in prevalence by age, but the former at consistently higher rates. Extrapolations might be biased substantially due to the higher pregnancy rates amongst uninfected individuals.

Conclusions: ANC-based data might draw a rather distorted picture of current dynamics of the HIV epidemic. Even though representing an obvious oversimplification, extrapolations of overall prevalence rates may correlate with that of the general population.

PIP: Assessments of the dynamics of the HIV/AIDS epidemic in Africa are based largely on sentinel surveillance of antenatal clinic (ANC) attenders. The validity of use of this key sentinel group in terms of HIV prevalence estimation was evaluated in a population-based survey of 4195 Zambian adults recruited through stratified random cluster sampling in urban Lusaka and rural Mposhi district in 1995-96. The survey was carried out in catchment populations of clinics used for national HIV surveillance. Overall, ANC surveillance tended to underestimate HIV prevalence in the general population, but differences were not statistically significant in either the urban or the rural area. In Lusaka, the adjusted overall HIV prevalence among ANC attenders was 24.4% compared with 26.0% in the population-based survey. The rural estimates were 12.5% and 16.4%, respectively. Comparison of age-specific prevalences indicated ANC overestimated infection in teenagers and underestimated HIV in those aged 30 years and over. Prevalence rates increased steeply by age, at comparable increment levels among women in both samples, while men had stable prevalence rates by age. These findings suggest that extrapolations from surveillance data may indeed produce a distorted picture of the current dynamics of the HIV epidemic in the population at large. Women with higher educational attainment are underrepresented in the ANC surveys. The higher pregnancy and birth rates among HIV-negative compared to HIV-positive women is another potential source of bias, as is the high rate of ANC clients aged 18-19 years (66%, compared with 34% in the general population of women). Information on single year of age, educational level, and residence should be collected in sentinel surveillance systems to allow appropriate stratification. Since HIV prevalence in young people may be the most reliable marker of changes in incidence, sample size increases in the 15-19 year age group should be considered.