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

2.2.2. Step 2: Estimate the size of key populations, if relevant
In each geographical unit, estimate the size of the key populations. The size of the populations most at risk
for HIV is one of the most critical determinants for:
• the likelihood of spread of HIV in an area where this population is prevalent
• the increase or decrease in the number of people with this behaviour
• how much the population contributes to new infections, particularly in low and concentrated epidemics.
For example, in a group of persons who inject drugs with a high frequency of injecting and who commonly
share needles when injecting, the potential for HIV to spread in a geographical unit is limited if the group
comprises only a handful of people.
Evaluating a National Surveillance System
In the case of sex work, the potential for spread depends both the number of sex workers and the number
of men who buy sex. Even if the number of sex workers is small, sex work can fuel an epidemic if the
percentage of men who buy sex is high, say more than 10% of the male population, and if sex is bought
frequently enough.
In a generalized epidemic, size estimates are needed for key populations whose behaviour puts them at risk
for HIV as well as for people who engage in high-risk sexual behaviour, such as having multiple partners,
concurrent partners, high levels of partner exchange and infrequent condom use.
The key populations in generalized epidemics can be measured, with some limitations, using nationally
representative surveys which include questions on behaviours.
Are highly precise population estimates needed
A highly precise estimate of population size is not needed to know roughly where most new infections come
from. It is more important to be able to distinguish sizes which differ by an order of magnitude or more.
That is, try to determine if the subpopulation at risk in a geographical unit is less than a hundred persons or
contains several thousand persons.
Knowing the relative size of key populations at higher risk in different geographical units is also helpful.
Because the population within a geographical unit is likely to be variable, it can be helpful to compare the
size of key populations at risk both as absolute numbers as well as a percentage of the general population.
When considering which subpopulation may be the source of more infections compared to another, consider
both the size of the key populations at higher risk and intensity of risk behaviour/potential for transmission.
This will require using the same population definitions to determine the size of the population and
characterizing the intensity of risk behaviours. As mentioned earlier, the impact of each subpopulation on
new infections in a given geographical unit should also take into account the potential number of infections
transmitted:
• to regular sexual partners of sex workers
• to clients of sex workers
• to men who have sex with men
• to persons who inject drugs.
There is no perfect method for estimating the size of populations at increased risk for HIV. Table 2.3 presents
a summary of methods with a short description. A guideline for estimating hidden populations has recently
been published. It provides technical details on various methods and tools (8).
13