Black African females aged 20-34 years (Spectrum estimate national level for 2012 at 1.37%).

In all countries included here, there has been a considerable decrease in modeled HIV incidence from peak levels, with variations in how much and at what rate this fall in modeled incidence may have occurred in different countries. Although HIV prevention progress can be assessed by a few measures of incidence reduction (percent change, value in 2012, rate of change), it is important to consider all of them on a country-bycountry basis, as only using one may provide the full picture of the progress that has been made in addressing the national HIV epidemic or its current state. High or low HIV incidence rates can translate into very different number of new HIV infections depending on the countries’ population sizes. Ethiopia had a very low incidence rate in 2012 (0.03%), but it still experienced an estimated 10,503 new infections that same year. Nigeria had a lower estimated incidence rate than Tanzania (0.23% vs. 0.32%), but it had nearly three times as many estimated new infections in 2012 (199,921 vs. 68,683). On the other hand, Swaziland, a much less populous country, had a high estimated incidence in 2012 (2.04%), but the number of new HIV cases in adults was much smaller at 10,287. Historical numbers of new HIV infections determine current ART needs and expenditures, while current (and future) new infections determine the additional, longterm treatment liabilities in countries.

Percent reduction of modeled incidence is another useful measure to assess progress in addressing the HIV epidemic, but it, too, cannot provide the whole picture. For example, while Ghana’s modeled incidence dropped 70%, from 0.18% to 0.05%, the slope of Botswana’s decline (2.23% to 1.25%) from 2004-2012 is steeper, despite a lower percent reduction in incidence (44%). Additionally, this measure is strongly influenced by its _historical_ incidence data and the height of the incidence peaks in the past, which can be problematic, as different versions of Spectrum can provide varying estimated levels, resulting in different percent reductions across software versions. When comparing longitudinal incidence trends using estimates from both 2010 and 2012 Spectrum software, it was found that incidence reduction was greater for some countries using 2010 Spectrum estimates (Burkina Faso, Central African Republic, Namibia, Rwanda, South Africa, Swaziland, Zimbabwe), and greater for others (Botswana, Congo, Ghana, Kenya, Lesotho, Malawi, Mozambique, Nigeria, Tanzania, Uganda, Zambia) using 2012 Spectrum (Fig. 1 and 2). These differences in incidence reduction comparing 2012 vs 2010 Spectrum estimates are quite extreme in some cases: Ghana (56% vs 17%), Nigeria (49% vs 3%), and Uganda (39% vs 4%).

If different Spectrum versions or the addition of one more year of data into the current Spectrum software (see Annex 2) result in different incidence estimates and historical peaks, this suggests some uncertainty in past estimations and future projections of HIV incidence, and complicates the assessment of the HIV epidemic. This is especially important as UNAIDS determines national progress using modeled HIV incidence changes, categorized by increasing incidence over 25%, stable incidence (less than 25% 48