The Spectrum program is used to estimate key HIV indicators from the trends in incidence and prevalence estimated by the Estimation and Projection Package or the Workbook. These indicators include the number of people living with HIV, new infections, AIDS deaths, AIDS orphans, the number of adults and children needing treatment, the need for prevention of mother-to-child transmission and the impact of antiretroviral treatment on survival. The UNAIDS Reference Group on Estimates, Models and Projections regularly reviews new data and information needs, and recommends updates to the methodology and assumptions used in Spectrum.

The latest update to Spectrum was used in the 2009 round of global estimates. This update contains new procedures for estimating: the age and sex distribution of adult incidence, new child infections occurring around delivery or through breastfeeding, the survival of children by timing of infection and the number of double orphans.

UNAIDS projections of adult HIV prevalence and incidence are prepared using the Estimation and Projection Package (EPP)

Spectrum is a modular program used to examine the consequences of current trends and future program interventions in reproductive health. The program and manuals are updated regularly and available in multiple languages free of charge at

The major inputs and outputs of the AIDS module of Spectrum are shown in

Inputs, outputs and process of the demographic and AIDS modules in Spectrum.

Several new features have been added to Spectrum for the 2009 round of estimates in response to new data, new treatment guidelines and new needs by national programmes for planning. There are new procedures for estimating the age and sex distribution of adult incidence, new child infections occurring around delivery or through breastfeeding, the survival of children by timing of infection, the effects of HIV infection in fertility and the number of double orphans.

EPP is used to fit a smooth prevalence trend to surveillance and survey data and to estimate the associated incidence among adults aged 15–49. Then, the incidence trend is passed to Spectrum. Since Spectrum calculations are disaggregated by age and sex, the adult incidence from EPP also needs to be converted to age- and sex-specific incidence. To do this, we used a mathematical method for estimating incidence by age from two cross-sectional measurements of prevalence.

The effects of ARV treatment are accounted for by removing the fraction of infected individuals that are alive due to treatment from the prevalence data in each survey.

These estimates of incidence are associated with uncertainty arising from the sampling error in the prevalence estimates. Further, in the countries with smaller epidemics, household surveys could underestimate the true HIV prevalence, given the concentration of infection in populations that are not fully captured (eg, hostels, brothels, military/police barracks).

We applied this method to data on HIV prevalence by age and sex from 28 DHS and AIDS Indicator Surveys. Four countries had more than one survey (Dominican Republic, Kenya, Mali and Zambia). For countries with adult prevalence above 4% the sample sizes are large enough for the incidence patterns to be relatively smooth.

Distribution of new HIV infections by age for males (left) and females (right) in high-prevalence countries. AIS, AIDS Indicators Survey; DHS, Demographic and Health Surveys; SBS, Sexual Behaviour Survey.

Patterns of the distribution of new HIV infections by age and type of epidemic. Conc, low level and concentrated epidemics; F, female; Gen, generalised epidemics; IDU, injecting-drug-user-driven epidemics; M, male.

The same analysis indicates that the ratio of female to male incidence among adults 15–49 varies from 0.5 to 2.4 with a median of 1.38 in generalised epidemics, 0.84 in low level and concentrated epidemics, and 0.42 in IDU-driven epidemics. The ratios vary considerably across countries. This variation is reflected when uncertainty analyses are run.

These patterns are used in Spectrum for all countries in each epidemic type and are assumed to be constant from the beginning of the epidemic. These assumptions produce a good match to the survey measures of prevalence by age and sex 20–30 years after the start of epidemic, as shown in

Comparison of the age pattern of prevalence from Spectrum projection from 1970 (dashed lines) to survey results (solid lines) in Malawi 2004 (left) and Swaziland 2006/2007 (right). DHS, Demographic and Health Surveys.

Previous versions of Spectrum estimated new HIV infections among children as occurring around delivery, through the combined probability of infection in utero, peripartum and intrapartum, and through breastfeeding. The survival pattern was modelled as the proportion of children infected at birth who survive to each future age and was based on fitting a double Weibull curve to longitudinal data.

In the latest version of Spectrum, infections around delivery are tracked separately from those occurring through breastfeeding in order to follow fast and slow progressors separately. The probability of HIV infection at birth for a child born to an HIV-positive mother is assumed to be 20% in the absence of prophylaxis, 11% with single-dose Nevirapine, 4% with dual ARV prophylaxis or 2% with triple ARV prophylaxis or treatment.

New survival curves for children infected at birth or through breastfeeding were constructed using data from 12 sub-Saharan African clinical trials and cohort studies.

HIV-infected children were divided into categories of early or late infection, and non-AIDS mortality was removed to focus on net AIDS mortality. Double Weibull curves were fitted to each set of data to determine the pattern of survival. As there was little follow-up beyond 2.5 years, there is limited information to inform the survival pattern beyond age 2.5. Therefore, the survival pattern was extended by assuming that children that have survived at least 2.5 years would have survival similar to young adults aged 15–24, with a median survival time of about 20 years.

Proportion of HIV-infected children surviving by time since infection for children infected around the time of birth (early infection, solid line) or later through breastfeeding (late infection, dashed line).

Spectrum estimates the number of maternal, paternal and double orphans by parent's cause of death (AIDS and non-AIDS) based on methods developed for Africa by Grassly and Timaeus in 2001.

The new analysis uses orphanhood data from 55 African DHS conducted in 29 countries, each of which collected data on 8000 to 33 000 children. This database does not represent a random sample of African countries or of points in time. Nevertheless, the surveys cover countries from all parts of the region. The only populous African country for which no data are available is the Democratic Republic of Congo.

The new model includes a somewhat simpler set of explanatory variables than the earlier regression model, requiring only children's age (_{k}_{t}^{2}_{k}^{2}_{t}_{i}

The coefficients of the revised model are shown in

Parameter estimates for random-intercept Poisson regression models of the percentage of double orphans among children aged 0–17 using Demographic and Health Survey data from countries in mainland sub-Saharan Africa with generalised HIV epidemics.

Variable | Coefficient | SE |

Intercept | 1.3478 | 0.1563 |

ln (percentage of children who are expected double orphans) | 0.7539 | 0.0283 |

Children's age | −0.02108 | 0.0131 |

ln (HIV prevalence in year | −0.1506 | −0.0540 |

Children's age×ln (HIV prevalence in year | 0.007225 | 0.0050 |

ln (HIV prevalence in year | 0.09887 | 0.0113 |

Intercountry variance of the intercept | 0.0196 | 0.0059 |

Intersurvey variance of the intercept | 0.0000 | 0.0000 |

Extra-Poisson variation | 1.7701 | 0.2071 |

Estimates of the excess risk of double orphanhood (with 95% CIs) from the random-intercept Poisson regression model at two levels of HIV prevalence for mainland African countries with generalised HIV epidemics

Children's age group (years) | 1% HIV prevalence | 25% HIV prevalence |

0–2 | 12.8 (12.3 to 13.3) | 10.9 (10.5 to 11.3) |

3–5 | 6.2 (6.0 to 6.4) | 5.7 (5.4 to 5.9) |

6–9 | 4.3 (4.1 to 4.5) | 4.2 (4.1 to 4.4) |

10–14 | 3.1 (3.0 to 3.2) | 3.4 (3.3 to 3.5) |

15–17 | 2.5 (2.4 to 2.6) | 3.0 (2.9 to 3.1) |

Compared with Spectrum's previous estimates, the revised model produces higher estimates of numbers of double orphans. The relative differences are greatest for groups in which numbers of double orphans are small (young children and populations with low HIV prevalence) and shrink to about 10% in populations with severe epidemics. The estimates are now more precise. Conditional on the accuracy of the estimates of maternal and paternal orphanhood and HIV prevalence generated by Spectrum, the regression model will usually predict the number of double orphans to within ±4%.

The validity of the predictions made by this regression model is contingent on the accuracy of the DHS data on orphans. However, the effects of under-reporting of double and single orphans tend to cancel out in the estimation of the coefficients, and various attempts to assess these data have only found evidence of small to moderate biases in them.

There are a number of other updates to Spectrum for the 2009 estimates. All the demographic inputs have been updated to use the 2008 round of estimates and projections from the United National Population Division.

Survival on ART has also been updated based on the latest studies for adults and children.

Spectrum is a tool that can be used by national programmes to estimate key planning indicators using their own data and the results from international studies. It incorporates the latest research findings and is regularly reviewed by the UNAIDS Reference Group on Estimates, Modelling and Projections. Modifications to Spectrum respond to new data availability and the needs of country programmes.

Many of the assumptions in Spectrum are derived from a small number of studies that may not be representative of all populations or all regions. Some estimates of the need for treatment rely on guidelines as they existed in 2008 and 2009, and ought to be updated to accommodate the new guidelines for treatment and PMTCT programmes. Spectrum calculations rely on incidence generated by another model, EPP, so it is important to ensure that the same assumptions are used in both models. It is impossible to validate Spectrum estimates for mortality, need for treatment or orphans for most countries because of a lack of survey or vital registration data with which to compare.

Spectrum is one of a package of tools that has been developed to assist national programmes and international organisations to estimate the impact of the AIDS epidemic and the needs for treatment and prophylaxis. It continues to be updated to incorporate the latest research findings and provide indicators needed for programme planning. Recent updates have improved the estimates, particularly for child infection, treatment need and double orphans.

The Spectrum software is used to calculate HIV indicators of interest on the basis of previously prepared prevalence estimates.

New calculations and assumptions have been added in order to incorporate the latest data and respond to the international commitment for expanded treatment.

New methods have been developed for estimating the age distribution of incidence, child survival and double orphans.

The AIM module in the Spectrum software and manuals and training materials were developed with funding from USAID (Geneva) and UNAIDS (Washington, DC).

None.

All authors reviewed the final manuscript. JS and PJ were primarily responsible for the introduction and the sections on uncertainty, strengths and weaknesses. Each of the other authors contributed a section of the paper: TH on methods for estimating incidence by age, MM and RB for estimates of child survival by mode of transmission, and IMT for updated methods for estimating double orphans.

Not commissioned; externally peer reviewed.