Human immunodeficiency virus (HIV) is a virus that attacks the immune system. The Centers for Disease Control and Prevention (CDC) estimates that 1.2 million people in the United States were living with HIV in 2018.

We often refer to HIV as a singular type of virus, but this is a little misleading. These viruses are actually incredibly diverse.

HIV can be divided into two main types: HIV-1 and HIV-2.

HIV-1 viruses can then be further categorized into different groups, subtypes, and strains.

While this may seem a little confusing, don’t worry. Below, we’ll break down exactly how HIV is classified, how testing works, and more.

Let’s dig a little deeper into HIV classification. We’ll do this by starting out with the two main types of HIV and then breaking each type down further into groups and, where applicable, subtypes.

Types

There are two different types of HIV. These are called HIV-1 and HIV-2. Although they may have similar names, these are actually two distinct types of virus. On a genetic level, HIV-2 is more than 55 percent different from HIV-1.

Overall, HIV-1 is the predominant type of HIV throughout the world. It’s estimated that 95 percent of people who are living with HIV have HIV-1.

HIV-2 is mainly found in western Africa. It doesn’t transmit from one person to another as well as HIV-1, and it may also progress more slowly.

HIV-2 is also resistant to some types of antiretroviral drugs, such as non-nucleoside reverse transcriptase inhibitors (NNRTIs).

Groups

HIV-1 viruses can be further classified into four different groups. These are:

  • Group M
  • Group N
  • Group O
  • Group P

Out of the four groups, viruses in Group M contribute to the majority of HIV cases worldwide. In fact, the “M” in Group M stands for “major.”

Viruses from the other three groups are much less common. They’re generally found in areas of central and western Africa.

HIV-2 viruses can also be divided into nine different groups, which are designated by the letters A through I. Groups A and D appear to be the only ones that are currently circulating in humans.

Subtypes

HIV-1 viruses within Group M are broken down even more into subtypes. These are groups of viruses that are genetically different from each other. There are currently nine different subtypes within Group M:

  • Subtype A
  • Subtype B
  • Subtype C
  • Subtype D
  • Subtype F
  • Subtype G
  • Subtype H
  • Subtype J
  • Subtype K

Genetically speaking, viruses from different subtypes can differ from each other by about 25 to 35 percent. Within a subtype, there’s still variation as well, but we’ll have more on that later.

With so much variation, you may be wondering if the subtypes differ in how they cause disease or how they’re transmitted. Researchers are currently working hard to answer these questions.

Strains

When a virus enters a host cell, it begins to replicate, meaning it makes more of itself. However, many viruses are very sloppy at making new copies of their genetic material. This means that mutations can occur.

All viruses mutate, and HIV is no exception. When viruses mutate, a different variant of a virus can be produced. Generally speaking, these variants can be referred to as viral strains.

For example, within Subtype B there can be a vast variety of strains that are slightly different from one another. However, they’re all still genetically similar enough to each other to be classified as Subtype B viruses.

It’s also possible that you may see the different subtypes referred to as strains. For example, an article may refer to “Subtype B strains.” This references all variants that are included under the genetic umbrella of Subtype B.

It’s possible to contract more than one strain of HIV. This is called superinfection. When superinfection occurs, the new strain can either replace or coexist in the body along with the original strain.

The exact prevalence of HIV superinfection is unknown, and estimates can vary based on individual studies. Some data suggest that the incidence rate of superinfection can range between 0 and 7.7 percent per year.

Superinfection can impact HIV treatment. This is because the new virus may be resistant to the antiretroviral drugs that a person is currently taking.

According to the CDC, superinfections that are difficult to treat with antiretroviral drugs are rare. Additionally, continuing to take antiretroviral drugs as directed can help to prevent a superinfection from occurring.

It’s also possible for a person to contract both HIV-1 and HIV-2. This dual infection has a prevalence of up to 3.2 percent in western Africa. Additionally, some data suggest that HIV-2 may slow HIV-1 progression.

We’ve covered a lot of information on the different kinds of HIV so far. The chart below summarizes some of what we’ve discussed.

HIV-1HIV-2
About this type of HIVHIV-1 is the main type of HIV worldwide. Most people that are living with HIV have HIV-1.HIV-2 is less common than HIV-1. It’s generally rare outside of western Africa, may progress more slowly than HIV-1, and is resistant to some antiretroviral drugs.
Does it have groups?Yes, there are four groups of HIV-1. They are:
Group M
Group N
Group O
Group P
Yes, there are nine groups of HIV-2. These are classified using the letters A through I.
Which group is most prevalent?HIV-1 viruses in Group M account for most cases worldwide.HIV-2 viruses in Groups A and D are the most prevalent.
Are there subtypes within these groups?Yes, Group M contains nine different subtypes. These are classified using letters A through K (without E and I). No, HIV-2 doesn’t have subtypes.
Which subtype is more prevalent?It depends on where you are. Subtype C is the most prevalent worldwide. However, Subtype B is most prevalent in the United States.Not applicable to HIV-2.
Is there anything else to know?Different subtypes from Group M can combine to form hybrid recombinant viruses, which can then go on to circulate within a population.It’s possible for an individual to contract both HIV-1 and HIV-2. In this case, HIV-2 may slow the progress of HIV-1.

You may now be wondering how HIV testing works in relation to the different kinds of HIV. Let’s take a look at the current CDC testing recommendations.

The first type of test that’s recommended is an antigen/antibody test. This test uses a small sample of blood to detect antibodies to HIV-1 and HIV-2 as well as an HIV-1 protein called p24.

However, this test just detects the presence of antibodies to both HIV-1 and HIV-2. It doesn’t indicate which type of virus is present. In order to do this, another test is needed to differentiate between the two types of antibodies.

Discovering whether an individual has contracted HIV-1 or HIV-2 is important. This is because treatment of HIV-2 can be different than HIV-1, as HIV-2 viruses are resistant to some types of antiretroviral drugs.

Drug resistance testing

Generally speaking, antiretroviral drugs work well against different groups and subtypes of HIV-1. However, some strains of HIV can be resistant to certain types of these medications.

Drug resistance testing is done prior to starting on antiretroviral drugs. This looks at different viral genes to detect mutations that can cause drug resistance. It can help a healthcare provider determine an effective treatment plan.

While there’s currently no cure for HIV, treatments have come an incredibly long way since the virus was first identified. Due to advances in treatment, people living with HIV can have long, healthy lives.

There are now many types of antiretroviral drugs available to treat HIV. According to the National Institutes of Health (NIH), taking antiretroviral medications each day as directed can reduce viral load to undetectable levels in 6 months or less.

Not only can having an undetectable viral load keep the immune system healthy, but it can also prevent transmission of HIV to others. People with an undetectable viral load have no risk of transmitting HIV to their partners via sex.

There are two different types of HIV. These are HIV-1 and HIV-2. Throughout the world, the majority of people living with HIV have HIV-1. HIV-2 is rare outside of western Africa.

HIV-1 viruses are very diverse. There are four groups of HIV-1. One group, Group M, causes most HIV-1 cases around the world. Viruses in Group M are further divided into nine subtypes, which can be made up of numerous variants, or strains.

Getting tested for HIV is important after a potential exposure. It’s also recommended for people that may be at a higher risk for contracting the virus. Antiretroviral medications can be used to effectively reduce virus to undetectable levels.

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