While often spoken of as a single entity, there are two distinct types of the human immunodeficiency virus (HIV): HIV-1 and HIV-2. Both are retroviruses, share similar transmission routes, and can lead to acquired immunodeficiency syndrome (AIDS) without treatment. The two types are genetically different, with more than 55% of their genetic material being distinct. These variations lead to significant differences in their spread, effect on the body, and medical management.
Geographical Distribution and Prevalence
The two types of HIV originated from separate zoonotic events, meaning they jumped from animals to humans. HIV-1 is understood to have crossed over from chimpanzees in Central Africa, while HIV-2 originated from the sooty mangabey monkey in West Africa. This difference in origin is a primary reason for their distinct geographical footprints. HIV-1 is the globally predominant type, responsible for about 95% of all infections worldwide.
In contrast, HIV-2 is much less common and remains largely concentrated in West Africa. While cases have been reported outside this region, they are often linked back to West Africa through travel or origin. The genetic diversity of HIV-1 is also far greater, with multiple subtypes circulating globally, whereas HIV-2 has fewer subtypes.
Differences in Transmission and Disease Progression
HIV-1 is more easily transmitted from person to person, largely because individuals with HIV-1 have a much higher viral load, which is the number of virus particles in the blood. Viral loads in people with HIV-1 can reach millions of copies per milliliter (mL) of blood, compared to under 10,000 copies/mL for those with HIV-2. This directly impacts the likelihood of passing the virus through shared needles, sexual contact, or from mother to child. The rate of mother-to-child transmission is 20 to 30 times lower for HIV-2 than for HIV-1, and sexual transmission is about five times lower.
The progression of the disease also differs significantly. People with HIV-2 experience a much slower disease course and may remain asymptomatic for many years. The advancement to AIDS is less frequent and slower than for those with HIV-1. This is because the body’s immune response is more effective at controlling HIV-2.
Diagnosis and Treatment Variances
Diagnosing HIV infection involves tests that detect antibodies produced by the immune system in response to the virus. While most initial screening tests can detect both HIV-1 and HIV-2, they often cannot distinguish between them. When a test comes back positive, a follow-up test called a differentiation assay is necessary to determine which specific virus is present.
This differentiation is important because treatment approaches are not the same for both viruses. While antiretroviral therapy (ART) is the standard of care for both, HIV-2 demonstrates natural resistance to certain classes of ART drugs. Specifically, HIV-2 is not susceptible to non-nucleoside reverse transcriptase inhibitors (NNRTIs), a common component in many HIV-1 treatment regimens. This inherent resistance means the selection of medications for a person with HIV-2 must be carefully considered and tailored. While treatment options for HIV-2 can be more limited, tailored ART can still effectively suppress the virus and allow for a healthy life.
Co-infection and Patient Outlook
Although rare, it is possible for an individual to be co-infected with both HIV-1 and HIV-2. This situation presents challenges for diagnosis and treatment, as the presence of both viruses complicates the clinical picture and the selection of an effective drug regimen. The rarity of co-infection means it is a less-studied area.
Due to its slower progression and lower transmission rates, the overall outlook for individuals with HIV-2 is more favorable than for those with HIV-1. Mortality rates are lower, and the period of asymptomatic infection is longer. However, once the disease does progress, the clinical outcome is similar for both types. With proper management and access to appropriate ART, individuals with either virus can effectively manage the virus and maintain their health over the long term.