HIV (human immunodeficiency virus) is spread through contact with genital fluids or blood of an infected person. The spread of HIV can occur when these secretions come in contact with tissues such as those lining the vagina, anal area, mouth, eyes (the mucus membranes), or with a break in the skin, such as from a cut or puncture by a needle.
During all stages of infection, billions of HIV particles (copies) are produced every day and circulate in the blood. This production of virus is associated with a decline (at an inconsistent rate) in the number of CD4 cells in the blood over the ensuing years.
CD4 cells are vital cells of the immune system, and their levels fall as the severity of the HIV infection increases. AIDS (acquired immunodeficiency syndrome) is the end result of unchecked HIV infection. In AIDS, the immune system collapses, opening the way for opportunistic infections and cancers to kill the patient.
Although the precise mechanism by which HIV infection results in CD4 cell decline is not known, it probably results from a direct effect of the virus on the cell as well as the body’s attempt to clear these infected cells from the system. In addition to virus in the blood, there is also virus throughout the body, especially in the lymph nodes, brain, and genital secretions.
The most common ways in which HIV is spreading throughout the world include sexual contact, IV drug abuse (through sharing needles), and by mother-to-child transmission during pregnancy, labor (the delivery process), or breastfeeding.
Sexual transmission of HIV has been described from men to men, men to women, women to men, and women to women through vaginal, anal, and oral sex. The best way to avoid sexual transmission is abstinence from sex until it is certain that both partners in a monogamous relationship are not HIV infected. Because the HIV antibody test can take weeks to turn positive after infection occurs, both partners would need to test negative for at least 12 and up to 24 weeks after their last potential exposure to HIV.
The next best method is the use of latex barriers. This involves placing a condom on the penis as soon as an erection is achieved in order to avoid exposure to pre-ejaculatory and ejaculatory fluids that contain infectious HIV. For oral sex, condoms should be used for fellatio (oral contact with the penis) and latex barriers (dental dams) for cunnilingus (oral contact with the vaginal area). A dental dam is any piece of latex that prevents vaginal secretions from coming in direct contact with the mouth. Although such dams occasionally can be purchased, they are most often created by cutting a square piece of latex from a condom.
Recent data has convincingly demonstrated that once a person has virologic suppression in blood after least six months of treatment, their likelihood of transmitting HIV to an uninfected partner, even without condoms, is virtually zero if they continue treatment.
The spread of HIV by exposure to infected blood usually results from sharing needles, as in those used for illicit opioid drugs. HIV also can be spread by sharing needles for anabolic steroids to increase muscle, tattooing, and body piercing.
To prevent the spread of HIV, as well as other diseases, including hepatitis, needles should never be shared. At the beginning of the HIV epidemic, many individuals acquired HIV infection from blood transfusions or blood products, such as those used for hemophiliacs. Currently, however, because blood is tested for both antibodies to HIV and the actual virus before transfusion, the risk of acquiring HIV from a blood transfusion in the United States is extremely small and is considered insignificant.
There is little evidence that HIV can be transferred by casual exposure, as might occur in a household setting. For example, unless there are open sores or blood in the mouth, kissing is generally considered not to be a risk factor for transmitting HIV. This is because saliva, in contrast to genital secretions, has been shown to contain very little HIV.
Still, theoretical risks are associated with the sharing of toothbrushes and shaving razors because they can cause bleeding, and blood can contain large amounts of HIV. Consequently, these items should not be shared with infected people.
Similarly, without sexual exposure or direct contact with blood, there is little if any risk of HIV contagion in the workplace or classroom.
Risk factors for acquiring HIV infection include increased amounts of virus in fluids and/or breaks in the skin or mucous membranes which also contain these fluids. This relates to the viral load in the infected person’s blood and genital fluids. If the viral load is high, the risk of transmitting HIV is also high. Conversely, those on effective antiretroviral therapy are less likely to transmit the virus to their partners. In fact, recent data has demonstrated that if a person’s plasma viral load is consistently undetectable for at least six months of therapy, the risk of transmitting HIV to their partner is essentially zero, leading to the phrase that “undetectable equals untransmittable.”
Another risk factor for HIV acquisition by a man is the presence of foreskin. This has most convincingly been demonstrated in high-risk heterosexual men in developing countries where the risk declines after adult male circumcision.
The risk of HIV transmission occurring after any potential exposure to bodily fluids is poorly defined.
The highest risk sexual activity, however, is thought to be receptive anal intercourse without a condom when the partner is not on antiretroviral therapy. In this case, the risk of infection may be as high as 3%-5% for each exposure. The risk is probably less for receptive vaginal intercourse without a condom and even less for oral sex without a latex barrier.
Despite the fact that no single sexual exposure carries a high risk of contagion, HIV infection can occur after even one sexual event. Thus, people must always be diligent in protecting themselves from potential infection.
Historically, the greatest successes in preventing viral illnesses have been the result of the development of preventative vaccines. Unfortunately, decades of research to develop an HIV vaccine has led to little hope for success. In 2007, a major setback in this area occurred when the STEP study investigating a promising vaccine candidate was prematurely stopped due to the lack of evidence that it produced any protection from HIV infection.
In contrast, a glimmer of hope did emerge with the report in 2009 of the results of the RV 144 Thai HIV vaccine trial, which demonstrated borderline effectiveness in the more than 16,000 recipients. While this vaccine demonstrated only limited evidence of protection, research is under way to further explore what can be learned for future vaccine development from this modest success.
Early advances in preventing HIV transmission resulted from educational programs describing how transmission occurs and providing barrier protection for those exposed to genital secretions and new needles or bleach to those exposed to blood by sharing needles. Despite these efforts, new infections in both the developed and developing worlds have continued at high rates. In light of the limited ability of counseling and testing to curb the spread of the HIV pandemic, many researchers have moved toward other biologic strategies for preventing HIV that do not rely solely on people changing their behavior. It is in this area where there has been some success.
During the last 10 years, there were several large studies showing that male circumcision along with behavioral counseling reduced the risk of heterosexual men acquiring HIV infection. This provides a novel prevention strategy for at-risk, HIV-uninfected heterosexual men.
Another major advance on the prevention front came from the HPTN 052 study in which HIV-infected individuals with CD4 cells between 350 cells/mm3 and 550 cells/mm3 who also had uninfected partners were randomly assigned to initiate antiviral therapy or wait until their CD4 cells declined to less than 250 cells/mm3 or they developed symptoms consistent with disease progression. All enrolled individuals were aggressively counseled about continued safe sex practices, provided condoms, and were monitored for sexual activities. The study ultimately showed that those treated early were more than 96% less likely to transmit to their partner than those who had antiviral treatment deferred. Subsequent cohort studies have shown that those who are virologically suppressed on antiretroviral therapy for at least six months have essentially no risk of transmitting to uninfected partners, even when not using condoms.
As opposed to treating infected people to protect their uninfected partners, another approach is to provide antiviral treatment to uninfected individuals, so-called pre-exposure prophylaxis (PrEP). The first success in this research arena came from the CAPRISA 004 study, which showed that vaginal administration before and after intercourse of a gel containing the antiretroviral agent tenofovir reduced the risk of transmission of both HIV and herpes simplex virus to heterosexual women. Other studies are under way to confirm the results of this study as well as to determine whether the results are any different if the agent is administered daily rather than simply around the time of intercourse. One such study was not be able to show that once-daily tenofovir vaginal gel demonstrated protection from infection compared to placebo gel. The reasons for this finding are not completely known, but it does appear that adherence with the therapy was very poor.
In 2010, the iPrEx study reported the results of the first large study testing the effectiveness of PrEP using orally administered therapy, as opposed to topical agents as in the vaginal PrEP studies. In this study, HIV-uninfected men who had sex with men who took TDF/FTC (tenofovir/Emtricitobine) once daily along with a comprehensive program to promote safe-sex practices and early treatment of sexually transmitted diseases experienced a markedly reduced risk of acquiring HIV compared with those receiving similar prevention practice without TDF/FTC. There are several other studies that have shown that once daily TDF or TDF/FTC have been effective for PrEP in heterosexual men, women, and intravenous drug users. Nevertheless, there are other studies of high-risk HIV-uninfected women that have shown no benefit, with convincing data in both studies demonstrating extremely low levels of treatment adherence with study medications. Based upon the data available, the United States FDA has approved TDF/FTC for use in high-risk HIV-uninfected individuals. When this therapy is utilized, it is clear that people need to be extensively counseled regarding the importance of continued use of condoms as well as diligent screening for HIV infection, acquisition of sexually transmitted diseases, as well as treatment adherence. Treated individuals also need to be made aware of potential side effects of treatment, including gastrointestinal symptoms, kidney damage, and decreases in bone mineral density.
Several novel strategies are being pursued to overcome the difficulty in getting people to adhere to PrEP. This includes studies to see whether treatment can be given less than daily, for example, around risk-taking activities. Other options include long-acting formulations, such as a vaginal ring impregnated with antiviral agents or long-acting intramuscular injections of CAB (cabotegravir), described above under new treatments that could be administered every few months.
A final prevention strategy of last resort is the use of antiretrovirals as post-exposure prophylaxis, so-called “PEP,” to prevent infection after a potential exposure to HIV-containing blood or genital secretions.
Animal studies and some human experience suggest that PEP may be effective in preventing HIV transmission, and it is based upon these limited data that current recommendations have been developed for health care workers and people in the community exposed to potentially infectious material. Current guidelines suggest that those experiencing a needle stick or who are sexually exposed to genital secretions of an HIV-infected person should take antiretrovirals for four weeks.
Those individuals considering this type of preventative treatment, however, must be aware that post-exposure treatment cannot be relied upon to prevent HIV infection. Moreover, such treatment is not always available at the time it is most needed and is probably best restricted to unusual and unexpected exposures, such as a broken condom during intercourse. If PEP is to be initiated, it should occur within hours of exposure and certainly within the first several days. Updated guidelines are published and available at https://aidsinfo.nih.gov/.
Medically Reviewed on 12/18/2019
DHHS Panel on Antiretroviral Guidelines for Adults and Adolescents. “Guidelines for the Use of Antiretroviral Agents in HIV-1 Infected Adults and Adolescents.” Washington D.C.: Department of Health and Human Services, 2018. <https://aidsinfo.nih.gov/contentfiles/lvguidelines/adultandadolescentgl.pdf>