Guidelines for second generation HIV surveillance - World Health ...

Weaknesses: Spectrum requires at least five data points over time to produce reasonably reliable prevalence
curves and related incidence curves. The Estimations and Projections Package (EPP) can only fit curves for
populations for whom there are trend data. Data requirements can be considerable. Most countries with
generalized epidemics tend to have good time-series data for ANC attendees but time-series data for other
populations that are important for the epidemic are generally not available.
Other incidence models
Modelling requires multiple time points of HIV prevalence and behavioural data for the different groups
present in a geographical unit. Some countries or areas have more data that can be used as inputs, for
example:
• HIV risk behaviour trends
• estimated sizes of populations at risk for HIV
• HIV prevalence trends
• data on the burden of STIs.
For some countries, more advanced modelling can be done, which not only provides incidence estimates
but also allows countries to project the expected trajectory of HIV incidence in different populations. These
are powerful planning tools. First, the UNAIDS Modes of Transmission tool is used to estimate the number
of new infections currently occurring among different populations. It does not project trends or trajectory
of incidence (28). In Kenya, for instance, heterosexual transmission was found to be the most prominent
mode of transmission in all geographical areas from both the epidemiological analysis and modelling
approach (29). Second, the Asian Epidemic Model (30) is a tool that can be used to predict trends for the
types of concentrated epidemics that occur in Asia. See the example that follows, which illustrates the Asian
Epidemic Model in Indonesia.
Strengths: These powerful planning tools allow countries to project forward on:
• the expected trajectory of HIV incidence
• the source of new HIV infections in different populations.
Weaknesses: Such tools require considerable amounts of data on the different populations and information
about their roles in the epidemic. Models have been developed, tested and validated only for specific
epidemic patterns. Also, trained staff is required to use the software.
Using the Asian Epidemic Model in Indonesia (Figure 4.6) (30)
Recall the Indonesia epidemiological zone map in Section 2 (Figure 2.3).
• The Asian Epidemic Model predicts that infections among clients of sex workers in Papua will grow
steadily and result in more infections among regular partners.
• Without saturation of prevention programmes, the number of new infections per year may double by
2020.
• Outside of Papua, the dynamics of the epidemic are more complex because of the interactions among
persons who inject drugs, men who have sex with men and female sex workers.
• New infections will increase sharply as the epidemic spreads beyond injection drug use and shifts
towards sexual transmission.
Ideally, models should not be used for monitoring trends as they are based on assumptions and are only
as good as the data that are used in the model. However, most countries do not have the data to measure
incidence and models are the only viable option.
HIV incidence assays
Assays and laboratory protocols for measuring HIV incidence rates directly at the population level have
been under development for a number of years. They are now beginning to be used by some countries
as a component of the HIV surveillance system. At present, these assays are not entirely accurate. They
require correction factors and further validation for use in different settings before they can be considered
as reliable surveillance tools (31,32). In the meantime, countries continue to rely heavily on tools that are well
tested and known to be both sensitive and specific for measuring prevalent infections.
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