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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Risk Factors

Estimated per act risk for acquisition
of HIV by exposure route[1]
Exposure Route Estimated infections
per 10,000 exposures
to an infected source
Blood Transfusion 9,000[2]
Childbirth 2,500[3]
Needle-sharing injection drug use 67[4]
Percutaneous needle stick 30[5]
Receptive anal intercourse* 50[6][7]
Insertive anal intercourse* 6.5[6][7]
Receptive penile-vaginal intercourse* 10[6][7][8]
Insertive penile-vaginal intercourse* 5[6][7]
Receptive oral intercourse 1[7]
Insertive oral intercourse* 0.5[7]§
* assuming no condom use </br> § source refers to oral intercourse
performed on a man

Sexual Contact

The majority of HIV infections are acquired through unprotected sexual relations between partners, one of whom has HIV. The primary mode of HIV infection worldwide is through sexual contact between members of the opposite sex.[9][10][11] Sexual transmission occurs with the contact between sexual secretions of one partner with the rectal, genital or oral mucous membranes of another. Unprotected receptive sexual acts are riskier than unprotected insertive sexual acts, with the risk for transmitting HIV from an infected partner to an uninfected partner through unprotected anal intercourse greater than the risk for transmission through vaginal intercourse or oral sex. Oral sex is not without its risks as HIV is transmissible through both insertive and receptive oral sex.[12] The risk of HIV transmission from exposure to saliva is considerably smaller than the risk from exposure to semen; contrary to popular belief, one would have to swallow liters of saliva from a carrier to run a significant risk of becoming infected.[13]

Approximately 30% of women in ten countries representing "diverse cultural, geographical and urban/rural settings" report that their first sexual experience was forced or coerced, making sexual violence a key driver of the HIV/AIDS pandemic.[14] Sexual assault greatly increases the risk of HIV transmission as protection is rarely employed and physical trauma to the vaginal cavity frequently occurs which facilitates the transmission of HIV.[15]

Sexually transmitted infections (STI) increase the risk of HIV transmission and infection because they cause the disruption of the normal epithelial barrier by genital ulceration and/or microulceration; and by accumulation of pools of HIV-susceptible or HIV-infected cells (lymphocytes and macrophages) in semen and vaginal secretions. Epidemiological studies from sub-Saharan Africa, Europe and North America have suggested that there is approximately a four times greater risk of becoming infected with HIV in the presence of a genital ulcer such as those caused by syphilis and/or chancroid. There is also a significant though lesser increased risk in the presence of STIs such as gonorrhea, Chlamydial infection and trichomoniasis which cause local accumulations of lymphocytes and macrophages.[16]

Infectivity seems to vary during the course of illness and is not constant between individuals. An undetectable plasma viral load does not necessarily indicate a low viral load in the seminal liquid or genital secretions. Each 10-fold increment of blood plasma HIV RNA is associated with an 81% increased rate of HIV transmission.[16][17] Women are more susceptible to HIV-1 infection due to hormonal changes, vaginal microbial ecology and physiology, and a higher prevalence of sexually transmitted diseases.[18][19] People who are infected with HIV can still be infected by other, more virulent strains.

During a sexual act, only male or female condoms can reduce the chances of infection with HIV and other STDs and the chances of becoming pregnant. The best evidence to date indicates that typical condom use reduces the risk of heterosexual HIV transmission by approximately 80% over the long-term, though the benefit is likely to be higher if condoms are used correctly on every occasion.[20] The effective use of condoms and screening of blood transfusion in North America, Western and Central Europe is credited with contributing to the low rates of AIDS in these regions. Promoting condom use, however, has often proved controversial and difficult. Many religious groups, most noticeably the Roman Catholic Church, have opposed the use of condoms on religious grounds, and have sometimes seen condom promotion as an affront to the promotion of marriage, monogamy and sexual morality. Defenders of the Catholic Church's role in AIDS and general STD prevention state that, while they may be against the use of contraception, they are strong advocates of abstinence outside marriage.[21] This attitude is also found among some health care providers and policy makers in sub-Saharan African nations, where HIV and AIDS prevalence is extremely high.[22] They also believe that the distribution and promotion of condoms is tantamount to promoting sex amongst the youth and sending the wrong message to uninfected individuals. However, no evidence has been produced that promotion of condom use increases sexual promiscuity,[23] and abstinence-only programs have been unsuccessful in the United States both in changing sexual behavior and in reducing HIV transmission.[24] Evaluations of several abstinence-only programs in the US showed a negative impact on the willingness of youths to use contraceptives, due to the emphasis on contraceptives' failure rates.[25] The male latex condom, if used correctly without oil-based lubricants, is the single most effective available technology to reduce the sexual transmission of HIV and other sexually transmitted infections. Manufacturers recommend that oil-based lubricants such as petroleum jelly, butter, and lard not be used with latex condoms, because they dissolve the latex, making the condoms porous. If necessary, manufacturers recommend using water-based lubricants. Oil-based lubricants can however be used with polyurethane condoms.[26] Latex condoms degrade over time, making them porous, which is why condoms have expiration dates. In Europe and the United States, condoms have to conform to European (EC 600) or American (D3492) standards to be considered protective against HIV transmission.

The female condom is an alternative to the male condom and is made from polyurethane, which allows it to be used in the presence of oil-based lubricants. They are larger than male condoms and have a stiffened ring-shaped opening, and are designed to be inserted into the vagina. The female condom contains an inner ring, which keeps the condom in place inside the vagina – inserting the female condom requires squeezing this ring. However, at present availability of female condoms is very low and the price remains prohibitive for many women. Preliminary studies suggest that, where female condoms are available, overall protected sexual acts increase relative to unprotected sexual acts, making them an important HIV prevention strategy.[27]

With consistent and correct use of condoms, there is a very low risk of HIV infection. Studies on couples where one partner is infected show that with consistent condom use, HIV infection rates for the uninfected partner are below 1% per year.[28]

In December 2006, the last of three large, randomized trials confirmed that male circumcision lowers the risk of HIV infection among heterosexual African men by around 50%. It is expected that this intervention will be actively promoted in many of the countries worst affected by HIV, although doing so will involve confronting a number of practical, cultural and attitudinal issues. Some experts fear that a lower perception of vulnerability among circumcised men may result in more sexual risk-taking behavior, thus negating its preventive effects.[29] Furthermore, South African medical experts are concerned that the repeated use of unsterilized blades in the ritual circumcision of adolescent boys may be spreading HIV.[30]

Prevention strategies are well-known in developed countries, however, recent epidemiological and behavioral studies in Europe and North America have suggested that a substantial minority of young people continue to engage in high-risk practices and that despite HIV/AIDS knowledge, young people underestimate their own risk of becoming infected with HIV.[31]

Exposure to Infected Body Fluids

CDC poster from 1989 highlighting the threat of AIDS associated with drug use

Needle sharing is the cause of one third of all new HIV-infections and 50% of hepatitis C infections in North America, China, and Eastern Europe. The risk of being infected with HIV from a single prick with a needle that has been used on an HIV-infected person is thought to be about 1 in 150 (see table above). Post-exposure prophylaxis with anti-HIV drugs can further reduce that small risk.[32] Health care workers (nurses, laboratory workers, doctors etc) are also concerned, although more rarely. This route can affect people who give and receive tattoos and piercings. Universal precautions are frequently not followed in both sub-Saharan Africa and much of Asia because of both a shortage of supplies and inadequate training. The WHO estimates that approximately 2.5% of all HIV infections in sub-Saharan Africa are transmitted through unsafe healthcare injections.[33] Because of this, the United Nations General Assembly, supported by universal medical opinion on the matter, has urged the nations of the world to implement universal precautions to prevent HIV transmission in health care settings.[34] Drug abuse has an additional effect of an increased tendency to engage in unprotected sexual intercourse.[35]

The risk of transmitting HIV to blood transfusion recipients is extremely low in developed countries where improved donor selection and HIV screening is performed. However, according to the WHO, the overwhelming majority of the world's population does not have access to safe blood and "between 5% and 10% of HIV infections worldwide are transmitted through the transfusion of infected blood and blood products".[36]

Medical workers who follow universal precautions or body-substance isolation, such as wearing latex gloves when giving injections and washing the hands frequently, can help prevent infection by HIV.

All AIDS-prevention organizations advise drug-users not to share needles and other material required to prepare and take drugs (including syringes, cotton balls, the spoons, water for diluting the drug, straws, crack pipes, etc). It is important that people use new or properly sterilized needles for each injection. Information on cleaning needles using bleach is available from health care and addiction professionals and from needle exchanges. In some developed countries, clean needles are available free in some cities, at needle exchanges or safe injection sites. Additionally, many nations have decriminalized needle possession and made it possible to buy injection equipment from pharmacists without a prescription.

Transmission of HIV between intravenous drug users has clearly decreased, and HIV transmission by blood transfusion has become quite rare in developed countries.

Mother-To-Child Transmission (MTCT)

In the absence of treatment, the transmission rate between the mother to the child during pregnancy, labor and delivery is 25%. However, when the mother has access to antiretroviral therapy and gives birth by caesarean section, the rate of transmission is just 1%.[3] A number of factors influence the risk of infection, particularly the viral load of the mother at birth (the higher the viral load, the higher the risk). Breastfeeding increases the risk of transmission by 4.04%.[37] This risk depends on clinical factors and may vary according to the pattern and duration of breast-feeding.[37]

Studies have shown that antiretroviral drugs, caesarean delivery and formula feeding reduce the chance of transmission of HIV from mother to child.[38] Current recommendations state that when replacement feeding is acceptable, feasible, affordable, sustainable and safe, HIV-infected mothers should avoid breast-feeding their infant. However, if this is not the case, exclusive breast-feeding is recommended during the first months of life and discontinued as soon as possible.[39] In 2005, around 700,000 children under 15 contracted HIV, mainly through MTCT, with 630,000 of these infections occurring in Africa.[40] Of the children currently living with HIV, almost 90% live in sub-Saharan Africa.

In Africa, the number of MTCT and the prevalence of AIDS is beginning to reverse decades of steady progress in child survival.[41] Countries such as Uganda are attempting to curb the MTCT epidemic by offering VCT (voluntary counseling and testing), PMTCT (prevention of mother-to-child transmission) and ANC (ante-natal care) services, which include the distribution of antiretroviral therapy.

References

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