The overall prevalence of HIV includes the number of diagnosed as well as undiagnosed people infected with HIV.
In the UK, estimates of undiagnosed infections rely on data from unlinked anonymous (UA) surveys from 3 selected adult populations:
- pregnant women
- people who inject drugs
- sexual health clinic attendees
- National Survey of Sexual Attitudes and Lifestyle (Natsal)
Data from the UA surveys and other surveillance and sexual behaviour systems are fed into a Multi-Parameter Evidence Synthesis (MPES) model which provides national prevalence estimates.
MPES model-based estimates of HIV prevalence
National estimates of the number of people living with HIV/AIDS (PLWHA) in the UK are obtained from a complex statistical model, fitted to a collection of census, surveillance and survey-type prevalence data. The model is based on a subdivision of the target UK population aged 15 years and above into key mutually exclusive exposure subgroups, and produces estimates by geographical region, infection diagnosis status and gender.
Unlinked anonymous (UA) surveys
The main objective of the UA programme is to monitor trends in the prevalence of both diagnosed and undiagnosed HIV infection in selected sub-populations. These surveys include not only individuals who have already been diagnosed with HIV infection, but also people who live unaware of their infection and would therefore not be captured by other surveillance systems.
UA survey of HIV prevalence in pregnant women survey
Pregnant women are an important group to target for HIV prevention as early diagnosis and appropriate management reduces mother to child transmission and improves the prognosis for the mother. Data from this group can also be used to contribute to the understanding of the burden of HIV in the general population in the UK.
The survey aims to :
- monitor progress in reducing mother to child transmission of HIV
- monitor seroprevalence of HIV among women giving birth in the UK
- enable estimation of the total number of HIV-infected persons living in the UK
The neonatal dried blood spot survey takes left-over dried blood spots taken from newborns around 10 days after birth and tests them for maternal HIV infection. This is a large seroprevalence survey which includes more than 60% of all births in England. Demographic data is collected with spots in selected regions through temporary linkage to birth registration records via the Office for National Statistics.
All samples are irreversibly unlinked and anonymised prior to HIV testing. The survey includes women whose HIV infection has been diagnosed as well as those unaware of their infection. This gives a measure of overall HIV prevalence in pregnant women.
This survey is conducted in collaboration with the Institute of Child Health, University College London.
More information on HIV in pregnant women.
Unlinked anonymous monitoring (UAM) survey of people who inject drugs (PWID)
The UAM survey of HIV and hepatitis among people who inject drugs (PWID) aims to measure the changing prevalence of HIV, hepatitis B and hepatitis C in PWID who are in contact with specialist drug services like needle and syringe programmes and treatment centres.
The programme also monitors levels of risk and protective behaviours among PWID. The data is used to assess and develop appropriate preventative and health education campaigns, evaluate the impact of such interventions, and to assist in the provision of services for PWID in the United Kingdom.
Public Health England (PHE), with support from Public Health Wales and Public Health Agency Northern Ireland, works in partnership with over 60 specialist drug agencies in England, Wales and Northern Ireland to gather the survey data.
PHE provides full training, all the survey materials needed, and feedback to each collaborating agency involved. Each year, the agencies are encouraged to ask all eligible clients that they see during their sampling period to participate in the survey. An eligible client being someone who is either currently injecting drugs or who has done so previously, and who has not already participated in the survey in the current calendar year.
Each eligible client is offered participation, and those who agree to participate are asked to complete a short questionnaire and to provide a dried blood spot sample. Identifying information is irreversibly unlinked from all samples before testing, ensuring that both the sample and the questionnaire are completely anonymous. Samples are tested for the presence of antibodies to HIV (signalling current infection), and antibodies to the hepatitis C and hepatitis B viruses (which can indicate current or previous infection).
The survey currently has 2 components.
- the main survey, aimed at people who inject psychoactive drugs, such as, heroin and crack cocaine.
- a sub-survey of people who inject image and performance enhancing drugs, such as, anabolic steroids and melanotan. This sub-survey runs every other year with participants recruited into each survey wave over an 18 month period.
In both of the questionnaires, participants are asked to provide information regarding their HIV and hepatitis C diagnosis status (if known), uptake of the vaccine against hepatitis B, their patterns of drug use (including treatment for drug dependency and participation in needle and syringe programmes) and their sexual behaviour. This information is used to assess the association between risky activities (such as needle sharing) and the prevalence of HIV and hepatitis C among PWID.
More information on PWID.
UA survey of genitourinary medicine (GUM) clinic attendees (GUM anon survey)
The GUM anon survey is the only survey in the UK to provide information on the prevalence of undiagnosed HIV among GUM attendees.
The GUM anon survey uses the unlinked anonymous technique on left-over specimens taken for routine tests to measure the HIV prevalence among GUM attendees not having a HIV test. The unlinked anonymous technique involves removing all identifying information from the specimens. Limited information on risk factors and whether voluntary confidential HIV testing was accepted is retained.
More information on the GUM anon survey.
Genitourinary medicine clinic activity dataset (GUMCADv2)
The GUMCADv2 is a pseudo-anonymised patient-level electronic dataset collecting information on diagnoses made and services provided by GUM clinics (Level 3) and other commissioned Level 2 (non-GUM) sexual health services. GUMCADv2 is an approved mandatory dataset.
More information on GUMCADv2.
National Survey of Sexual Attitudes and Lifestyle (Natsal)
The British National Surveys of Sexual Attitudes and Lifestyles, or Natsal, are among the largest and most detailed studies of sexual behaviour in the world.
Three Natsal surveys have taken place:
- Natsal-1 in 1990 to 1991
- Natsal-2 in 1999 to 2001
- Natsal-3 in 2010 to 2012
The project is being managed by a multi-disciplinary team of researchers from University College London (UCL), the London School of Hygiene & Tropical Medicine (LSHTM) and NatCen Social Research (NatCen).
More information on Natsal.