The Human Immunodeficiency Virus (HIV) causes Acquired Immunodeficiency Syndrome (AIDS), a condition that weakens the body’s immune system. Unlike many viruses, HIV is not a single, uniform entity. It exists in various forms, often referred to as strains, demonstrating significant genetic diversity. This diversity plays a considerable role in its global impact and the challenges associated with treatment and vaccine development.
The Two Major HIV Types
HIV is broadly categorized into two main types: HIV-1 and HIV-2. HIV-1 is by far the most common type worldwide, responsible for most infections globally. It is highly virulent and progresses to AIDS more rapidly than HIV-2, typically within 8-10 years without treatment. HIV-1 also exhibits a higher transmission rate, making it the dominant form of the epidemic.
In contrast, HIV-2 is less common and primarily concentrated in West Africa, though cases have been identified elsewhere. HIV-2 generally causes a milder form of the disease, with a slower progression to AIDS, often over 15-20 years. Its transmission efficiency is also lower compared to HIV-1, which contributes to its more limited geographical spread. Both types target CD4+ T-cells, but their genetic makeup and disease progression patterns differ.
HIV Groups and Subtypes
HIV-1 classifies into four distinct groups: M, N, O, and P. Group M, also known as the “main” group, is responsible for over 90% of all HIV-1 infections globally, driving the vast majority of the pandemic. The other groups (N, O, and P) are much rarer and are primarily found in specific regions of West-Central Africa.
Group M itself is further divided into multiple genetically distinct subtypes, often designated by letters (e.g., A, B, C). These subtypes show varying geographical distributions; for example, subtype B is prevalent in North America, Western Europe, and Australia, while subtype C is dominant in Southern Africa and India. Subtype A is common in Eastern Europe and Central Asia. The genetic differences among these subtypes can influence disease progression and responsiveness to certain treatments.
Circulating Recombinant Forms (CRFs) arise when two different HIV subtypes infect the same person and exchange genetic material during replication. This process creates a new, hybrid virus that can then be transmitted to others. CRFs are increasingly common and complicate global HIV epidemiology and the development of effective interventions.
Drug-Resistant Strains
Drug-resistant HIV strains emerge when the virus undergoes genetic mutations that allow it to replicate even in the presence of antiretroviral therapy (ART) medications. This phenomenon often occurs if a person living with HIV does not consistently adhere to their prescribed ART regimen. When medication levels are insufficient, the virus can replicate unchecked, and random mutations that confer resistance to the drugs may arise and become the dominant viral population. These mutations alter the virus’s targets, preventing the drugs from binding effectively or inhibiting viral processes.
Drug resistance can be either acquired or transmitted. Acquired resistance develops due to incomplete adherence to ART, leading to the selection of resistant viral variants. Transmitted drug resistance occurs when a person is infected with an HIV strain already resistant to one or more antiretroviral drugs. This can complicate initial treatment choices, necessitating resistance testing before starting therapy.
HIV Superinfection
HIV superinfection describes a rare but significant event where an individual already living with one strain of HIV becomes infected with a second, genetically distinct strain. This is different from the continuous mutation and evolution of the initial strain within a person’s body. The second infection introduces new viral genetic material into the host, potentially altering the course of the existing infection.
HIV superinfection can have considerable consequences. A person might acquire a strain that is resistant to their current antiretroviral medications, making their treatment regimen less effective. The newly acquired strain could also be more aggressive or have different biological characteristics, potentially accelerating the progression of immune system damage. While not common, superinfection highlights the importance of consistent prevention measures even after an initial HIV diagnosis.