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Surveillance of HIV infection

UNAIDS/WHO Working Group on Global HIV/AIDS and STI Surveillance

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Measuring the prevalence and incidence of HIV

PREVALENCE: Rate of HIV in a defined population. Period Prevalence: Rate of HIV over a

specified period of time (usually 1 year) Point Prevalence: Rate of HIV infections in

as short a period as possible (1-2 months) INCIDENCE: Rate of new HIV infections

over a specified period of time (usually 1 year)

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Objectives of HIV Surveillance To assess the HIV seroprevalence in the population or in

population groups To monitor trends of HIV infection over time and place To provide baseline information for estimates and future

projections of HIV infection and AIDS To obtain, reinforce or increase the commitment of policy

makers, health workers, local and international groups and all sectors in AIDS prevention and care programs

To provide baseline data for appropriate planning of health and medical services.

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HIV Sentinel Surveillance

Repeated cross-sectional HIV prevalence studies in selected population groups at selected sites.

Trends of HIV infection are monitored over time, by group and by place or site.

Results can be applied confidently only to the selected population and sites surveyed.

Community(population)-based (e.g.:CSW, IVDU, MSM)

Clinic/health facility based (e.g.: ANC, STI, TB)

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Advantages

Monitors trends of infection in a chosen population Can be successfully carried out among high-risk population groups even when HIV infection in the general population is very low. Can conveniently choose high-risk and low-risk groups for study and follow-up. Less expensive to conduct than general population surveys. The process can become “routine” over a period of time. No participation bias as it is done in an unlinked anonymous manner.

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Disadvantages

Results from studies of sentinel groups cannot be applied to the general population.

Results from sentinel sites can be considered representative only of the population utilizing the services of the sentinel site.

Results could still be biased due to non-participation of sentinel group members (i.e. selective access to health facilities).

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Service-based surveillance

Unlinked anonymous testing using sample collected for other purposes in selected health facilitates

No need for informed consent, minimises participation bias, reduced cost.

Services must be available with sufficient coverage

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Community-based surveillance

When there are no services available or blood is regularly collected

Need for informed consent and counselling if the infected are informed

Potential participation and selection biases (reduced if saliva or urine are collected)

Potential impact on prevention services Community involvement and support are

essential

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Potential Sentinel Groups

• STD Clinic Attendees• Commercial Sex Workers (Male and Female)• Male homosexuals and bisexuals• Intravenous• Multiple Blood Recipients• Frequent Travellers• Prisoners

Moderate to High Risk of HIV Infection

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Potential Sentinel Groups

• Antenatal Clinic Attendees (Pregnant Women)• Voluntary Blood Donors• Health Care Workers• Factory Workers• Persons taking patients to clinics• Newborns• Military/Police Recruits• Adult Medical Outpatients• TB patients• Participants in surveillance of other diseases

Low Risk of HIV Infection

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HIV Surveillance in Groups Representing the General Population

HIV prevalence in groups representative of adults of sexually active age in the general population

Most useful in countries with generalized epidemics

Useful not only for trend analysis but also for HIV prevalence estimates

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Antenatal Clinics (ANC)

The most common Sentinel Population Not perfectly representative of all

women and even less of men Importance of the coverage of ANC

services (>80% in Africa, much less in Asia and Latin America)

Importance of geographic coverage (All areas? Urban/rural?)

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Biases with ANC data Only pregnant women are tested (HIV reduces

fertility) Only pregnant women who attend ANC are

tested Clinics selected may not be representative In general terms, ANC data:

underestimate prevalence in general female population

overestimate prevalence in the rural population

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Issues with ANC surveillance

Importance of younger age groups (15-24 yrs)

Consecutive sampling Time frame, point prevalence Sample sizes Socio-demographic variables testing strategies (Unlinked, voluntary)

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Representative male groups Military recruits

Blood normally taken Unlinked testing Recruitment process (random, universal, voluntary) Data from only a very limited age group

Screening for occupational health Factory workers Pre-employment screening Migrant workers Insurance

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HIV sero-surveys in the general population

In theory, the best method to obtain a reliable estimate of HIV prevalence in the general population

Normally quite expensive, difficult to conduct and presenting serious ethical problems

Requires informed consent and counselling Would be useful from time to time to “calibrate”

regular HIV surveillance (males/females ratio, urban/rural)

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ce HIV testing can be added to other population-based studies conducted for other public health objectives (e.g.: DHS+, HBV, Malaria, anaemia)

Most of the cost and logistics problems already included in the original study design.

Consistent sampling frame If appropriate samples are already being collected,

unlinked anonymous testing is still possible.

Potential negative impact on the original objectives of the study. Only feasible when and where these studies are conducted

HIV sero-surveys in the general population

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Blood donors The main goal is to screen all donations for

blood safety All population is tested HIV data is available at no additional cost Several biases:

Selected groups Self deferral Multiple donations HIV+ informed and removed

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Blood donors

Difference between:

Difference between

Donors = individuals Donations= blood bags

Paid donors Voluntary donors Replacement donors

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HIV Surveillance in populations at high risk

HIV surveillance in sub-populations whose behaviour may carry a higher risk than average of HIV infection

Most useful for concentrated or low epidemics

Mainly for trend analysis Limited use for prevalence or impact

assessment

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Challenges

Many risk behaviours are highly stigmatised and some are illegal

Little support for intervention in these groups

Hard to reach populations Anonymity or confidentiality is

essential in order to avoid negative effects on prevention efforts

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Testing, Screening, Surveillance• TESTING

The application of an HIV antibody test to determine if an individual is positive or negative for HIV antibody. (Voluntary testing or case detection).

• SCREENINGThe systematic application of HIV antibody test to a population of apparently healthy people for the purpose of detecting the number of people (or blood samples) infected with HIV. The primary aim is not to diagnose HIV infection in a specific person (Blood donors)

• SURVEILLANCEThe collection of information of sufficient accuracy and completeness on the distribution and spread of infection to be pertinent to the design, implementation or monitoring of prevention and care activities. Since it is not feasible to collect information from the total population, surveillance will have to rely on routine collection of data from sentinel groups.

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HIV Testing

• VOLUNTARY CONFIDENTIAL TESTING• Requires informed consent and counselling• Participation bias is likely

• VOLUNTARY ANANYMOUS TESTING• Requires informed consent and counselling• Coded sample, only the patient can link the results• Participation bias is possible

• UNLINKED ANANYMOUS TESTING• Testing of blood collected for other purposes• No coding, no consent, no counselling required• Participation bias minimized

• MANDATORY TESTING• Testing required for benefit/service/employment (blood donors)• Participation bias possible

• COMPULSORY TESTING• Testing is forced on the individual (Unethical)• Can be anonymous or confidential• Participation bias may still occur

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Unlinked Anonymous Testing

• Uses blood which is collected for other purposes (Testing of blood samples, not individuals)

• Ensures anonymity• Avoids the need for informed consent and counselling• Minimizes participation bias• More practical to implement

LIMITATIONS

• Detailed data on high-risk behaviours and other important variables cannot be obtained

• Only groups that have blood taken for other purposes can be studied• HIV-infected persons cannot be contacted and informed about their

status

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Selection of Sentinel Sites

• Sites where blood is already being drawn for other purposes

• Representative of high-risk and low-risk groups and/or areas

• Accessible and convenient• Sufficient number of patients• Staff willing to participate in surveillance activity

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Data Analysis• For meaningful interpretation, results of HIV sentinel

surveillance cannot be aggregated. Prevalence rates should be calculated separately per site and sentinel group.

• Confidence Intervals (CI), a statistical measure of the precision of the prevalence estimate, should be calculated for a predetermined degree of accuracy (at least 90%).

• Sentinel surveillance data should be used to monitor HIV trends over time. Results of HIV sentinel surveillance do not provide an accurate estimate of HIV prevalence in a population or population group.

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Confidence Intervals

0

3

6

9

12

15

18

• Trends in HIV prevalence in STD patients in one sentinel site with 90% CI.

• Significant increase in prevalence from 1991 to 1992

• Apparent (non-significant) increase from 1992 to 1993.

• Larger CI in 1993 due to small sample size.

1991 1992 1993

Number tested 412 413 227

Number HIV+ 24 56 34

HIV Prevalence 5.8 13.8 14.9

90% C.I. 1.9 2.8 4