1st August 2023

Background

Human immunodeficiency virus (HIV) was first identified in the 1980s. Despite the remarkable progress in treatment and prevention strategies, it remains endemic in many places worldwide, including Eastern Europe, Asia and Sub-Saharan Africa. It is a major cause of morbidity and mortality worldwide and a key global public health concern. The prognosis of HIV has hugely improved since the virus was first identified, and it is now a manageable chronic disease. This is primarily due to the advent of antiretroviral therapy (ART).

Pathophysiology and virology

HIV is a retrovirus. Retroviruses infiltrate the cells of host organisms and use the enzyme reverse transcriptase to transcribe their viral RNA into DNA, which is then incorporated into the host cell genome. Once the virus has ‘taken over’ the host cell, it replicates, creating more HIV-infected cells that proliferate throughout the host organism.

HIV infects CD4 receptor-positive white blood cells. These are T-helper cells which communicate with antigen-presenting cells and are a key part of the cell-mediated immune response. Infection of these cells with HIV, therefore, weakens the immune system.

The amount of HIV-infected cells in the blood is measured as ‘viral load’ – the higher the viral load, the fewer healthy CD4 cells, and the greater burden of disease.

There are two types of HIV – HIV-1 and HIV-2. HIV-1 is the most common, with HIV-2 mainly confined to West Africa. HIV-1 is divided into four distinct strains – group M is by far the most common, with N, O and P much rarer.

Epidemiology

As of 2022, WHO estimates that 37.7 million people are infected with HIV worldwide, with Sub-Saharan Africa most affected.

Advances in treatment mean that survival rates of HIV are increasing, meaning that in countries such as the UK, where treatment is widely available, prevalence has increased whilst mortality decreases – people are living longer with the disease. Undiagnosed HIV (approximately one-quarter of UK cases) is the commonest cause of mortality.

Certain behaviours and conditions increase the risk of contracting HIV, which are as follows:

• Unprotected anal or vaginal sex
• Co-morbid sexually transmitted infections
• The use and sharing of contaminated needles and syringes for injecting intravenous drugs
• Needle stick injuries

HIV is transmitted via bodily fluids, such as blood, breast milk, semen, and vaginal secretions. HIV can also be transmitted vertically (from mother to child, during pregnancy or childbirth). The most common routes of spread are via sexual intercourse and through blood.

When first identified, HIV was thought of as a disease which affected the male homosexual population. In the UK, gay or bisexual men, Black Africans, and people born abroad are the three groups most at risk of HIV. However, heterosexual transmission is the most common cause of infection worldwide.

Prevention

Prevention is a key strategy for managing the rate of HIV infections. Risk factor reduction includes sexual health awareness and the provision of condoms to reduce infection risk, testing and counselling for those diagnosed with risk conditions such as STIs and tuberculosis, needle exchange programmes, and the use of antiretroviral drugs as post-exposure prophylaxis (PEP) for those potentially exposed to HIV.

PEP is offered to people who may have been exposed through sexual contact, needle sharing, or through occupational exposure – and has an evidence basis for reducing the risk of infection if started within 72 hours of potential exposure. Treatment consists of a four week course of dual therapy with the antiretrovirals tenofovir/emtricitabine and raltegravir.