Changes in HIV incidence or prevalence in a cohort or a population

In order to get round the statistical power problem, HIV incidence in the whole of a defined at-risk population ‘before and after’ the introduction of an intervention can sometimes be used. Indeed, this may be the only ethical way of measuring the efficacy of an intervention where it would be unethical to withhold it from the control arm. An example is condom- or syringe-exchange schemes.

However, it is much more difficult to tease apart the range of factors which may be responsible for changes in incidence over time within a whole population, than to compare incidence between the two arms of a randomised trial.

For example, were the early reductions in HIV incidence in populations exposed to HIV, as among gay men during the 1980s, Ugandans in the early 1990s and Zimbabweans in the early 2000s, a consequence of changes in behaviour or an inevitable feature of the normal pattern of an epidemic? While it is clear from international data that mass adoption of condom use and early safer-sex campaigns coincided with rapid falls in sexually transmitted infections in gay men, and that HIV incidence peaked in gay communities in 1983/84, the subsequent decline in HIV incidence may be attributable to a declining number of men in the primary phase of infection capable of transmitting HIV easily to their partners.1

Another example concerns the apparently paradoxical results of the adoption of injection-equipment exchange in Canada in the 1990s.

Studies have generally demonstrated a decline in incidence among intravenous drug users (IDUs) when equipment exchange is introduced. However, a study in Montreal, Canada,2 demonstrated a greater risk of seroconversion amongst syringe-exchange users than IDUs not attending needle exchange during a mean follow-up period of 15 months (33 versus 13%).

However, another study of all needle exchanges in the Canadian city of Vancouver3 showed that needle exchanges tended to attract those injecting drug users already identified by other studies as being at highest risk – unstable, high-frequency injectors with multiple risks, including sex work, unprotected sex with other IDUs, ‘polydrug’ use (the use of more than one drug) and high frequency of sharing with strangers. Injecting drug users who used syringe exchanges less frequently (less than once a week) were less likely to share these characteristics. These two studies of syringe exchange illustrate the danger of jumping to unwarranted conclusions solely on the basis of incidence data in a cohort.

Decreases in HIV prevalence amongst some segments of the population may indicate that HIV-prevention efforts are succeeding in a broad sense, while an increase in prevalence may give an indication of increasing transmission rates. However, it is hard to correlate such changes with prevention programmes unless a long time span is being used.

For example, it is reasonable to argue that a fall in HIV prevalence amongst Ugandan women attending antenatal clinics for the birth of their first child suggests that prevention efforts have reduced HIV prevalence in that country, since this group of women are likely to have become sexually active fairly recently. But the fall in prevalence amongst the whole population in the first- and hardest-hit region of Uganda was only marginally to do with behavioural changes during the 1990s and was more to do with attrition due to AIDS deaths.4,5


  1. Catania J et al. Changes in condom use among homosexual men in San Francisco. Health Psychology, 10, 190-199, 1991
  2. Bruneau J et al. Increased HIV seroprevalence and seroincidence associated with participation in needle exchange program. Eleventh International Conference on AIDS, Vancouver, abstract TuC.323, 1996
  3. Archibald C et al. Needle exchange program attracts high risk injection drug users. Eleventh International Conference on AIDS, Vancouver, abstract TuC.320, 1996
  4. Asiimwe–Okiror G Declines in HIV prevalence in Ugandan pregnant women and its relationship to HIV incidence and risk reduction. Eleventh International Conference on AIDS, Vancouver, abstract MoC905, 1996
  5. Wawer MJ et al. Declines in HIV Prevalence in Uganda: Not as Simple as ABC. Twelfth Conference on Retroviruses and Opportunistic Infections, Boston, abstract LB27, 2005
This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.
Community Consensus Statement on Access to HIV Treatment and its Use for Prevention

Together, we can make it happen

We can end HIV soon if people have equal access to HIV drugs as treatment and as PrEP, and have free choice over whether to take them.

Launched today, the Community Consensus Statement is a basic set of principles aimed at making sure that happens.

The Community Consensus Statement is a joint initiative of AVAC, EATG, MSMGF, GNP+, HIV i-Base, the International HIV/AIDS Alliance, ITPC and NAM/aidsmap

This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.

NAM’s information is intended to support, rather than replace, consultation with a healthcare professional. Talk to your doctor or another member of your healthcare team for advice tailored to your situation.