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

Figure A.2 provides trend data for a rapid decline in prevalence observed among a population of persons
who inject drugs between 1998 and 2007. It is not unusual to see trends like this from surveillance data
among persons who inject drugs. Among them, HIV prevalence may rapidly rise or fall in a short period of
time. Yet, even with greatly reduced transmission, it is unlikely that HIV prevalence could drop this quickly.
When you see a trend like this, you have to consider the natural and unnatural factors that affect prevalence
trends, as discussed below.
Figure A.2. Dramatic decline in HIV prevalence among persons who inject drugs, 1997 to 2007
0.80
0.70
HIV prevalence
0.60
0.50
0.40
0.30
0.20
% HIV positive
0.10
0.00
1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
Possible explanations for trend in Figure A.2
Natural causes
• Dramatic drop in HIV incidence
• Saturation of the population with HIV infection and no new susceptible persons
• Mortality among HIV-infected persons who inject drugs
• A combination of these factors might explain part of the decline, but they could not explain all of it.
Unnatural causes
• Excluding people who tested positive in the previous round of surveillance (common source of error)
• Change in the site (for example, from facility-based persons who inject drugs to community-based
persons who inject drugs)
• Large influx of HIV-negative persons who inject drugs.
In Figure A.3, sentinel surveillance data for persons who inject drugs in several regions of the country are
given. In one region, HIV prevalence appears to have decreased steadily for a number of years, but reversed
and increased dramatically in the last round of surveillance. Could this recent increase reflect an explosive
epidemic among persons who inject drugs in this region
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