The epidemiology of Human immunodeficiency virus 1 (HIV-1) is the study of the distribution and the factors that affect the virus and the disease that it causes, AIDS, in various groups and places. HIV-1 is a retrovirus that infects human cells and causes progressive immunodeficiency, increasing susceptibility to opportunistic infections and cancers.

HIV-1 is transmitted through contact with infected body fluids, including breast milk, blood, semen, and vaginal secretions. Sexual contact, mother-to-child transmission, drug injection use, and risky medical procedures are the primary means of transmission.  

The World Health Organisation (WHO) estimates that an estimated in 2020, 36.2 million persons worldwide were infected with HIV-1, making up the 37.7 million people living with the virus. Most HIV-1 cases are concentrated in sub-Saharan Africa, which accounted for 69% of the global total in 2020. Other regions with a high prevalence of HIV-1 include Latin America, the Caribbean, Eastern Europe, and Central Asia.  

There are four categories for HIV-1: M (main), O (outlier), N (non-M/non-O), & P. Within group M, there are nine subtypes (A-D, F-H, J, and K) and several circulating recombinant forms (CRFs) that result from recombination between different subtypes. The distribution of HIV-1 subtypes and CRFs varies geographically and over time.

The most typical subtype worldwide is C, followed by subtype A and CRF02_AG. Subtype B is predominant in North America, Western Europe, Australia, and Japan. Subtype C dominates in Southern Africa, Eastern Africa, India, and Nepal. In West and Central Africa, CRF02_AG is typical.  

The epidemiology of HIV-1 is influenced by several variables, including the virus’ basic properties, the host immune response, the availability and effectiveness of antiretroviral therapy (ART), the social and behavioral determinants of transmission, and the interactions with other infections.

For example, some studies have suggested that HIV-1 infection may increase the risk of acquiring or transmitting another gonorrhoea, chlamydia, syphilis, and other STIs such the HSV-2 herpes simplex virus, as well as the human papillomavirus (HPV). Conversely, some STIs may facilitate HIV-1 transmission by causing genital ulcers or inflammation, increasing viral shedding or susceptibility.

Another example is the coinfection of HIV-1 with hepatitis B virus (HBV), which can affect both infections’ clinical course and treatment outcomes. The epidemiology of HIV-1 is also dynamic and evolving. The introduction of ART in the mid-1990s has significantly increased the survival and standard of living of people with HIV-1. However, ART poses challenges such as drug resistance, adverse effects, adherence issues, and cost.

Moreover, ART may have unintended consequences on the transmission dynamics of HIV-1 by reducing mortality and morbidity among infected individuals, increasing their sexual activity or risk behaviors, or altering their viral load or infectiousness. Therefore, monitoring the trends and patterns of HIV-1 infection and disease in different settings and populations is essential to inform prevention and control strategies. 

Classification and Structure 

• Kingdom:     virus 
• Phylum:        Artverviricota 
• Class:              Revtraviricetes 
• Order:           Ortervirales 
• Family:         Retroviridae 
• Subfamily:    Orthoretrovirinae 
• Genus:           Lentivirus 
• Species: Human immunodeficiency virus 1 (HIV-1) 

The structure of Human immunodeficiency virus 1 (HIV-1) can be distilled into the following five points: 

• HIV-1 is a retrovirus that belongs to the genus Lentivirus and the family Retroviridae. Its envelope is made of the host cell membrane. 
• containing two viral glycoproteins: gp120 and gp41. 
• Underneath the envelope is a protein matrix composed of p17, which supports the structure and shape of the virion. 
• Inside the matrix is a conical capsid made of p24, which encloses two copies of positive-sense single-stranded RNA and several enzymes, such as reverse transcriptase, integrase, and protease. 
• The RNA genome of HIV-1 is about 9.7 kb long and contains Nine genes: nef, vif, vpr, vpu, pol, env, tat, and rev (or sometimes tev). These genes encode 19 proteins involved in various aspects of the viral life cycle. 
• The RNA genome also has several structural features, such as a 5′ cap, a 3′ poly(A) tail, long terminal repeats (LTRs), trans-activation response element (TAR), rev response element (RRE), packaging signal (Psi), splice donor and acceptor sites, central polypurine tract (cPPT), central termination sequence (CTS), and adverse regulatory element (NRE). 

The antigenic types of human immunodeficiency virus 1 (HIV-1) are the different variants of the virus that have distinct antigenic properties, meaning that they can elicit different immune responses from the host. Antigenic types are essential for understanding the diversity, evolution, transmission, and pathogenesis of HIV-1, as well as for developing vaccines and diagnostics. 

HIV-1 is classified into four groups: M (primary), N (non-M/non-O), O (outlier), & P. Each group is thought to represent a separate transmission of simian immunodeficiency virus (SIV) into humans. Group M is the most prevalent and diverse group, accounting for more than 90% of HIV-1 infections worldwide.

Nine subtypes (A–D, F–H, J, and K) make up Group M in total) and several circulating recombinant forms (CRFs) that result from recombination between different subtypes. The distribution of HIV-1 subtypes and CRFs varies geographically and over time. Subtype B is predominant in North America, Western Europe, Australia, and Japan. Subtype C dominates in Southern Africa, Eastern Africa, India, and Nepal. CRF02_AG is common in West and Central Africa.  

The antigenic properties of HIV-1 are mainly determined by the envelope glycoprotein gp120, which binds to the CD4 receptor and a coreceptor (CCR5 or CXCR4) on the surface of target cells. Gp120 comprises five variable regions (V1-V5) and five conserved regions (C1-C5). The variable regions are responsible for the antigenic diversity of HIV-1, while the conserved regions are involved in the coreceptor binding and conformational changes of gp120.

The antigenic types of HIV-1 differ in their gp120 sequences, structures, and functions. For example, some antigenic types have a higher affinity for CCR5 or CXCR4 than others, which affects their tropism and pathogenicity. Some antigenic types can also escape neutralizing antibodies by changing their surface antigens or hiding within infected cells. 

The pathogenesis of HIV-1 infection involves several key steps: 

• Entry to the cell: HIV-1 binds to specific receptors on immune cells, fuses with their membranes, and releases its genetic material into the cell. 
• Replication and transcription: The virus converts its RNA into DNA, integrates this DNA into the host cell’s genome, and replicates itself within the cell. 
• Recombination: High mutation rates and recombination between different HIV-1 strains lead to genetic diversity and the emergence of new variants. 
• Assembly and release: new virus particles form at the cell membrane, bud off, and mature to become infectious. 
• Spread within the body: HIV-1 can spread through direct contact or body fluids, infecting various cells found in various organs and tissues, including the central nervous system. 
• Genetic variability: HIV-1 exhibits extensive genetic diversity, with different subtypes and recombinant forms prevalent in different regions. 
• Immune response and evasion: The host immune system fights HIV-1, but the virus can evade immune responses through various mechanisms, including antigenic variation and suppression. 
• Disease progression: HIV-1 infection progresses through stages, including acute infection, clinical latency, symptomatic infection, and AIDS, with disease severity influenced by viral load, CD4+ T cell count, and other factors. 

Host defenses against HIV-1 infection can be categorized into innate and adaptive immunity. Innate immunity involves physical barriers and cellular/molecular components that detect and respond to the virus, but HIV-1 can evade these defenses through various strategies.

On the other hand, adaptive immunity includes humoral (antibody) and cellular (T cell) responses that are specific and memory-based, but HIV-1 can also evade these defenses through antigen mutations and immune depletion. The outcome of this dynamic interaction between host defenses and HIV-1 determines the course and severity of the infection.

Factors like viral load, viral subtype, host genetics, coinfections, nutrition, healthcare access, and adherence to antiretroviral therapy (ART) influence the effectiveness of these defenses. ART is the primary treatment for HIV-1, which can suppress viral replication and restore immune function but cannot eliminate the virus from the body, necessitating lifelong treatment to prevent disease progression. 

HIV type 1 (HIV-1) infection results in many clinical manifestations. Some of the possible signs and symptoms include: 

• Fever 
• Headache 
• Muscle aches and joint pain 
• Rash 
• Sore throat and painful mouth sores 
• Swollen lymph glands, mainly on the neck 

Rapid weight loss, extreme exhaustion, mouth or genital ulcers, fevers, night sweats, and skin discolorations are some of the symptoms of AIDS. People frequently develop cancer and other ailments with AIDS, which can cause additional symptoms. 

Diagnosis of HIV-1: 

• Antigen/Antibody Test: Common and effective, detects HIV-1 p24 antigen and antibodies. A confirmatory test follows a positive result. 
• Nucleic Acid Test (NAT): Detects HIV-1 RNA directly, highly sensitive, used for early detection, confirming positive antibody tests, and monitoring viral load. 
• p24 Antigen Test: Less sensitive, not for routine diagnosis, but can aid in acute cases or infant screening. 

Criteria for HIV-1 Diagnosis: 

• Confirmed Infection: Positive antigen/antibody test + confirmatory test OR positive NAT. 
• Probable Infection: Positive antigen/antibody test without confirmation OR positive p24 antigen test without an antibody test. 

AIDS Diagnosis: 

• Confirmed or probable HIV-1 infection + AIDS-defining illnesses OR CD4 count < 200 cells/microliter. 

Ways to prevent HIV-1 infection: 

• Condom Use: Consistent and correct condom use during sex creates a barrier against HIV transmission. 
• Antiretroviral Medicines: 
• PrEP: Daily pill for high-risk individuals to reduce HIV transmission risk. 
• PEP: Short-term drugs within 72 hours of potential exposure. 
• ART: For those with HIV, suppresses the virus, improving health and preventing transmission. 
• Regular HIV Testing: Knowing your status helps you make informed decisions and seek care if needed. 
• Safe Sexual Practices: Choose lower-risk sexual activities and limit sexual partners. Faithfulness and testing are essential. 
• Avoid Sharing Drug Equipment: Do not share needles or syringes when using drugs. Use sterile equipment and seek help for substance abuse. 

These measures are crucial for HIV prevention and protection. 

• Antigenic Properties of the Human Immunodeficiency Virus Envelope Glycoprotein Gp120 on Virions Bound to Target Cells | PLOS Pathogens 
• 10.6C: The Life Cycle of HIV – Biology LibreTexts