How to understand HIV/AIDS estimates ? (Q&A)

[Guest guest]

Q & A on HIV/AIDS estimates

Understanding the latest estimates of the 2006 report on the global

AIDS epidemic

Part one: The data

1. What data do UNAIDS and WHO base their HIV prevalence estimates on?

2. What are the strengths and weaknesses of using antenatal and

household surveys in estimating HIV infection levels for generalized epidemics?

3. Are population-based surveys more accurate than antenatal surveys?

4. Which are the more accurate sources of data: sentinel surveillance

or case reporting?

Part Two: From the data to the estimates

5. How are the HIV/AIDS estimates arrived at?

6. Can the new estimates be compared with those from 2005?

7. Why have UNAIDS and WHO changed the age group ranges?

8. Why are UNAIDS and WHO publicizing ranges of HIV and AIDS estimates?

9. If UNAIDS and WHO claim the current estimates are more accurate,

why are the ranges for some countries so large?

10. How confident are UNAIDS and WHO about the estimates of the

number of people who die of AIDS each year?

11. What is being done to improve national HIV estimates?

Part three: Interpreting the new estimates

12. Is the AIDS epidemic slowing down?

13. Has the epidemic peaked in sub-Saharan Africa?

13.1 What might be causing the apparent stabilization of HIV

prevalence in sub-Saharan Africa?

13.2 There is no such thing as the " African " AIDS epidemic

14. Is the percentage of women infected with HIV rising globally?

15. Are more young people becoming infected with HIV?

___________________

Understanding the latest estimates of the 2006 report on the global

AIDS epidemic

Part one: The data

1. What data do UNAIDS and WHO base their HIV prevalence estimates on?

The precise numbers of people living with HIV, people who have been

newly infected or who have died of AIDS are not known. Achieving 100% certainty

about the numbers of people living with HIV globally, for example, would

require testing every person in the world for HIV

every year—which is logistically impossible and pose ethical

problems. But we can estimate those numbers by using other sources of data.

UNAIDS/WHO estimates are based on all pertinent, available data—

including surveys of pregnant women attending antenatal clinics,

population-based surveys (conducted at the household level), sentinel

surveillance among populations at higher risk of HIV infection,

case reporting, vital registration systems (the official recording of

births and deaths), as well as other surveillance information.

Different sets of data are used to calculate estimates of HIV

prevalence for generalized (high-level - where adult HIV prevalence among the

general adult population is at least 1% and transmission is mostly

heterosexual) and concentrated (low-level - where HIV is

concentrated in groups with behaviours that expose them to a high

risk of HIV infection) epidemics.

In countries with generalized epidemics, estimates of HIV prevalence

are primarily based on surveillance among pregnant women attending sentinel

antenatal clinics (ANC). Such data are collected on an annual basis and are

currently our primary basis for the assessment of

trends. If available, population-based sample surveys that include

testing for HIV infection (conducted much less frequently) are used to improve

the antenatal clinic data-based

estimates

For countries with low-level or concentrated epidemics, HIV estimates

are based on studies among key populations who are at higher risk of HIV

exposure—such as injecting drug users, sex workers, or men who have sex with

men.

Countries with concentrated epidemics sometimes have additional

sources of data which can help refine estimates. In countries such as Argentina

and Brazil, which have extensive voluntary counselling and testing programmes,

case reports can add to the estimation

process and make estimates more precise.

Better data from country surveillance and steady improvements in the

modelling methodology are enabling UNAIDS/WHO to develop more accurate

estimates.

2. What are the strengths and weaknesses of using antenatal and

household surveys in estimating HIV infection levels for generalized epidemics?

Each of these methods has its strengths and weaknesses. Generally,

estimates based on antenatal clinic attendees provide a good indication of HIV

infection trends among 15–49 year-olds over time.

Studies have shown that high proportions of women in most of the

highly-affected countries have access to antenatal clinic services. Where

possible, estimates derived from antenatal clinic data have been compared at

local level with HIV prevalence data acquired in

community- based surveys. Such validation exercises have concluded

that estimates based on antenatal clinic sentinel surveillance provide a good

approximation of HIV prevalence among adults aged 15-49 (men and women combined)

in the local community. However,

ANC surveillance is limited in that it only samples pregnant women

attending public health services and therefore excludes women who are not

pregnant or sexually active and who do not attend public health clinics. The

most important limitation is often related to the selection of sentinel

antenatal clinics. In general, clinics with larger volumes of pregnant women

are needed to obtain a minimum of 300 samples during the 4-6 week annual survey

of sentinel

clinics. Such clinics are more likely to be in urban areas, and the

sample of clinics is often not geographically representative. Remote rural

clinics are underrepresented for the most part, although countries are

increasingly trying to increase their representation as in-country surveillance

efforts are expanding.

National population-based household surveys, on the other hand, can

reveal important information about the national prevalence level and about the

spread of HIV, particularly among young people, men and residents in rural

areas. However, population-based surveys by their nature exclude certain

high-risk populations (e.g., people living in hostels, army recruits, etc.) and

might therefore underestimate HIV prevalence. Non-response due to absence from

households and refusing to participate in these surveys could also lead to bias

in the HIV estimates. Current research, however, indicates that in most

instances these biases are relatively small.

Population-based surveys are costly, complex undertakings, especially

if biological testing is included. Therefore, they are done with long intervals

in-between and few countries have done more than one national survey with HIV

testing since 2000.

Considered together, the various data can yield more accurate

estimates of HIV infection levels and the demographic impact of AIDS. However,

HIV and AIDS estimates (whether derived from household surveys or sentinel

surveillance data) need to be assessed carefully,

and the data and assumptions reviewed continually.

3. Are population-based surveys more accurate than antenatal surveys?

For all diseases a sound population-based sample provides better

estimates of disease prevalence than a clinic-based sample. National

population-based surveys reveal important information about the national

prevalence level and about the spread of HIV, particularly

among young people, men and residents in rural areas. If response

rates are good (e.g. over 75%) and there is no evidence of systematic biases of

exclusion of a large proportion of the population with likely different levels

HIV infection, then national estimates that consider data from all sources

(surveillance, population-based surveys and if available mortality data) should

be close to the household survey result.

4. Which are the more accurate sources of data: sentinel surveillance

or case reporting?

Case reporting generally tends to substantially underestimate the

number of people living with HIV. Most countries that rely on case reporting

focus the data collection on specific atrisk groups, often missing other groups.

Often, case reporting tends to focus heavily on injecting drug users, and often

the data collected reflect trends only among those users who interact with

government authorities (for example, by being arrested or attending drug

treatment clinics).

However, in countries that have extensive voluntary counselling and

testing programmes (such as Argentina and Brazil), case reports may enable more

precise estimates to be developed. Nonetheless, case reporting is unlikely to

capture people living with HIV who were recently infected, and who therefore

present no symptoms of infection. For these reasons, case reports can only

indicate the minimum number of people living with HIV.

On the other hand, reliance on sentinel surveillance of at-risk

groups can lead to overestimation of HIV prevalence in these groups.

This is because such surveillance in some cases detects HIV infection rates

among individuals who are at highest risk of HIV infection. For example,

sentinel surveillance among sex workers or their clients often focuses on those

who seek treatment at sexually transmitted infection clinics—and who, by

definition, have had unprotected sex.

However, other sex workers and clients who do practise safe sex—and who

therefore tend not to present at these clinics with sexually

transmitted infections—generally are not captured in this surveillance.

Part Two: From the data to the estimates

5. How are the HIV/AIDS estimates arrived at?

UNAIDS and WHO, in close consultation with countries, employ a six-

step method to obtain estimates of HIV prevalence for men and women. An

increasing number of countries have adopted these methods to develop national

estimates.

Different approaches are used for generalized epidemics (where adult

HIV prevalence exceeds 1% and transmission is mostly heterosexual) and low-level

or concentrated epidemics (where HIV is concentrated in groups with behaviours

that expose them to a high risk of HIV infection).

Between March 2005 and April 2006, UNAIDS and WHO conducted 12

regional workshops, training national personnel/ technicians from over 150

countries responsible for HIV estimates in the specific tools and methodologies

used to produce the national estimates in

this report. In addition UNAIDS and WHO participated in 10 country-

specific consensus meetings on HIV estimates. These methods allow for

standardization in measurement methods and allow cross-national comparisons and

regional aggregation and estimates.

Details on the six-step method and selected references to the wide

range of papers published by leading scientists from around the world, UNAIDS

and WHO can be found in Annex A.

6. Can the new estimates be compared with those from 2005?

The latest estimates cannot be compared directly with estimates

published in previous years. Nor should these latest estimates be compared

directly with those UNAIDS/WHO will publish in the years to come.

Why not? Because the assumptions, methodologies and data used to produce the

estimates are gradually changing as a result of ongoing enhancement of our

knowledge of the epidemic. Comparing the latest estimates with those published

in previous years is liable to yield misleading conclusions.

In a nutshell, the latest estimates—for the current year and for past

years—will tend to be more accurate and reliable than those produced in previous

years, since they are based on improved methods and more data than earlier

estimates. This kind of caution is not unusual

when dealing with global estimates of disease.

7. Why have UNAIDS and WHO changed the age group ranges?

The new estimates of the number of adults living with HIV (and of

adults with new infections and of AIDS mortality) are no longer restricted to

those in the 15–49 age group. Historically,

UNAIDS and WHO restricted the estimates to this age group to ensure

comparability across countries, especially for HIV prevalence.

However, it has become evident that a substantial proportion of people living

with HIV are 50 years and older, as shown

in age distributions of HIV and AIDS case reports, community studies and

population–based surveys. Accordingly, UNAIDS and WHO now present estimates of

adults living with HIV, new infections and AIDS deaths among adults for all

adults `15 years and older'. In addition, we continue to provide estimates of

HIV prevalence for `adults 15–49 years', to continue to allow for comparisons

across countries. UNAIDS and WHO also estimate trends among children `less than

15 years of age'.

8. Why are UNAIDS and WHO publicizing ranges of HIV and AIDS estimates?

The ranges reflect the degree of uncertainty associated with estimates and

define the boundaries within which the actual numbers lie.

In earlier UNAIDS/WHO reports, we reported point estimates (for

example, fixing HIV prevalence in country X at 12.5%). In addition, we also

published the ranges of uncertainty around those point estimates, depending on

the quality of the data that had yielded the

estimates. This was done because all estimates were associated with

some level of uncertainty.

Because the quality of data varies from country to country, the

ranges of uncertainty surrounding our estimates can widen or narrow depending on

the country. As well, presenting point estimates might have encouraged a false

sense of precision, notwithstanding the fact that ranges of uncertainty were

also provided.

Improved methods, enhanced data and new estimation tools are enabling

a better understanding of the degrees of uncertainty that surround HIV and AIDS

estimates. This is part of an ongoing process of improving estimates and

developing appropriate ranges—all of

which are vital for effective HIV/AIDS planning and programming at

national and regional levels.

UNAIDS and WHO are confident that the actual numbers of people living

with HIV, people who have been newly infected or who have died of AIDS lie

within the reported ranges.

9. If UNAIDS and WHO claim the current estimates are more accurate,

why are the ranges for some countries so large?

The ranges reflect the degrees of uncertainty around HIV estimates in

particular countries. Accordingly, the ranges vary, depending on the quality of

HIV data available in different countries.

Four factors determine the extent of the ranges around the HIV

estimates:

(i) The HIV prevalence level – Ranges tend to be smaller when HIV

prevalence is higher. Thus the bounds around the best estimate of adults living

with HIV in Zambia are relatively small (1,100,000 – 1,200,000) while they are

much wider in a lower prevalence country such as Djibouti (3,900 – 31,000).

(ii) The quality of the data – Countries with better quality data

have smaller ranges than countries with poorer quality data. The ranges for Asia

and the Pacific are comparatively broad—which reflects the fact that HIV

surveillance of key populations (such as injecting drug users, sex workers and

men who have sex with men) is relatively poor in most countries in that region,

hence resulting in

more uncertainty. In general, the ranges for sub-Saharan Africa are

narrower, because of recent improvements in the collection and interpretation of

HIV data in that region.

(iii) The number of steps or assumptions used to arrive at an

estimate – The more steps and assumptions, the wider the uncertainty range is

likely to be (since each step introduces additional uncertainties). For example,

ranges around estimates of adult HIV prevalence are smaller than those around

estimates of HIV incidence

among children, which require additional data on the probability of

mother-to-child HIV transmission. The latter are based on prevalence among

pregnant women, the probability of mother-to-child HIV transmission, and

estimated survival times for HIV-positive children. There is therefore greater

uncertainty in these estimates than for adult prevalence alone.

(iv) The type of epidemic (generalized or low-level/concentrated) –

Ranges tend to be wider in countries with low-level or concentrated epidemics

than in countries with generalized epidemics because in low-level or

concentrated epidemics, one needs to estimate both the numbers of people in the

groups at higher risk of HIV infection and HIV prevalence rates in those groups.

10. How confident are UNAIDS and WHO about the estimates of the

number of people who die of AIDS each year?

Estimates of adult AIDS mortality are based on several assumptions

and additional sets of data—including estimates of the numbers of adults and

children who are HIV-infected, and estimations of survival times from infection

with HIV to death for adults and children infected with HIV.

Civil registration systems are the best source to obtain an estimate

of the mortality due to AIDS. However, in most countries with generalized

epidemics, coverage of civil registration is too low to provide useful

information on AIDS mortality. Some countries have local demographic

surveillance or general information on adult mortality from censuses and surveys

that can help estimate mortality levels due to AIDS.

Estimating mortality in countries with low-level or concentrated

epidemics is even more difficult. Some at-risk groups are likely to have

different background mortality, in other words they are more prone to other

causes of death (for example, injecting drug users are

vulnerable to fatal drug overdoses and other life-threatening hazards). All this

can have substantial effects on patterns of mortality.

Unfortunately, country- specific data on mortality and on changes in risk

behaviour are seldom available. However, some countries with lowlevel/

concentrated epidemics have well-functioning vital registration systems that

include the cause of death.

11. What is being done to improve national HIV estimates?

New and different sources of data, such as national population-based

surveys, are enabling more accurate estimates and more refined understandings of

the epidemic's trends.

Importantly, the roles of national AIDS programmes have changed

significantly since the first set of UNAIDS/WHO country specific estimates was

produced in 1997.

Initially, countries were requested to comment on provisional estimates. The

extent of their involvement has increased subsequently

In the past several years, for example, UNAIDS and WHO, along with

their technical partners (including East-West Center, Family Health

International, The Futures Group, the US Census Bureau and the US Centers for

Disease Control and Prevention), carried out a series of regional training

workshops in which epidemiologists from over 150 countries were trained in the

HIV estimation process.

Such efforts have led to much greater involvement by national

programmes, national statistics offices and other government and academic

organizations in the production of estimates. The result has been better quality

estimates, due to the use of additional data

and the application of local knowledge.

UNAIDS and WHO continue to work with countries, partner organizations

and experts to improve data collection. These efforts will ensure that the best

possible estimates are available to assist governments, non-governmental

organizations and others in gauging the status of the epidemic and monitoring

the effectiveness of prevention and care efforts.

Part three: Interpreting the new estimates

12. Is the AIDS epidemic slowing down?

We can say that the epidemic is slowing down globally, but not all

countries are experiencing a decline in new HIV infections. Globally, the HIV

incidence rate (the annual number of new HIV infections as a percent of

previously uninfected persons) is believed to have peaked in the late 1990s and

to have stabilized subsequently. However, the

numbers of people living with HIV have continued to rise, due to population

growth and, more recently, the lifeprolonging effects of antiretroviral therapy.

13. Has the epidemic peaked in sub-Saharan Africa?

UNAIDS Secretariat and WHO analyses over the past five years show

that the epidemic is levelling off in much of sub-Saharan Africa, albeit at

unacceptably high levels. However, it is important to realise that a stable

prevalence is only possible if HIV- associated deaths are replaced by new

infections. Thus, a stable prevalence in sub-Saharan Africa still represents

nearly 3 million new infections each year.

The number of people living with HIV in the region rose dramatically

in the late 1980s and 1990s, and was still growing in 2005, although at a slower

rate. This slower growth is a result of a peak in new infections which occurred

in the late-1990s and an increase in the

annual number of people who die of AIDS.

It is now increasingly clear that across most of sub-Saharan Africa

(including parts of southern Africa), HIV prevalence among pregnant women

attending antenatal clinics has been roughly level for several years—albeit at

very high levels in Southern Africa. This apparent `levelling off' of HIV

prevalence has been interpreted by some observers as an indication that the

AIDS epidemic might have reached a turning point in sub-Saharan Africa.

Unfortunately, available evidence does not offer grounds for such

conclusions.

Even though HIV prevalence rates have stabilized in this region the

actual number of people infected continues to grow because of population growth.

Applying the same prevalence rate to a growing population will result in

increasing numbers of people living with HIV.

13.1 What might be causing the apparent stabilization of HIV

prevalence in sub-Saharan Africa?

Two factors are causing the apparent stabilization or " levelling off "

of prevalence rates observed in much of the region: AIDS mortality rates and HIV

incidence. High and, in some countries, rising rates of AIDS mortality and

continuing high HIV incidence offsetting this mortality are the cause of this

appearance of levelling off. In Zambia, for example, national HIV prevalence

appears to have stayed relatively stable for the past 8–10 years. Since it is

estimated that close to 100,000 people living in Zambia have been newly

infected annually over that period, overall prevalence has remained level

because AIDS has killed about the same number of people each year. HIV

prevalence might therefore appear stable, but it hides a persistently high

number of annual, new HIV infections and an equally high number of AIDS deaths.

On the other hand, as ARV therapy coverage increases, it is possible

that we will see prevalence levels increase as more people infected

with HIV live longer.

But we are not, unfortunately, witnessing a region-wide decline in the

epidemic. In the absence of effective interventions, the epidemic will

continue to wreak havoc in these countries.

13.2 There is no such thing as the " African " AIDS epidemic

It is important to remember that there is not one, typical " African "

AIDS epidemic. In ten countries, adult HIV prevalence is below 2%, while in five

other countries it is over 20%. These extreme differences in prevalence levels

fall roughly into geographically separate areas.

Eight countries of southern Africa (Botswana, Lesotho, Mozambique,

Namibia, South Africa, Swaziland, Zambia, Zimbabwe) have

prevalence above 15%, with Swaziland having prevalence over 30%.

In West Africa, HIV prevalence is much lower with no country having a

prevalence above 10% and most having prevalence between one and

five percent. Adult prevalence in countries in Central and East Africa

falls somewhere between these two groups.

The countries of the region also differ in the time course of their

epidemics, with epidemics starting earlier in East and Central Africa

and much later in countries in southern Africa. Uganda is an example

where adult HIV prevalence peaked in the early 1990s.

14. Is the percentage of women infected with HIV rising globally?

No, this estimate is roughly the same as it was in 2001. UNAIDS/WHO

estimate that almost half (49%) of adults aged 15 years and over living with HIV

are women. However, the numbers of women living with HIV globally are rising.

This is because the overall numbers of

people living with HIV are rising. By far the majority of women

living with HIV are in sub- Saharan Africa. In sub-Saharan Africa, young women

are much more likely to be HIVinfected than young men – a ratio of 3 to 1.

The concentration of HIV-infected children in sub-Saharan Africa

reflects the estimate that 59% of HIV-infected adults in that region are women

and that women in Africa have more children than elsewhere in the world. No

other region proportionately has as many women

living with HIV. However, in the Caribbean women already account for

nearly half of the adults living with HIV.

15. Are more young people becoming infected with HIV?

UNAIDS estimates that about half of all new HIV infections worldwide

are among children and young people up to 25 years.

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Additional sources of information:

For a technical description of the processes used to arrive at the

estimates and further explanation on the quality of HIV, please see:

N, Grassly NC, Garnett GP, Stanecki KA, Ghys PD (2004)

Estimating the global burden of HIV/AIDS: What do we really know about the HIV

pandemic? Lancet 2004:336, 26 June 2004.

For a detailed description of the methods, software, quality of data

and development of ranges, please see the series of seven articles published in

the journal Sexually Transmitted Infections the second week of July 2004. The

articles can be downloaded for free from their

website at www.Sti.bmjjournal.com.

More information on the UNAIDS Reference Group on Estimates,

Modelling and Projections can be found at www.epidem.org.

UNAIDS/WHO Last updated: June 2007

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