The modes of transmission of HIV are similar to those of hepatitis B, in particular with respect to sexual, parenteral, and vertical transmission. Although certain sexual practices (eg, receptive anal intercourse) are significantly riskier than other sexual practices (eg, oral sex), it is difficult to quantify per-contact risks. The reason is that studies of sexual transmission of HIV show that most people at risk for HIV infection engage in a variety of sexual practices and have sex with multiple persons, only some of whom may actually be HIV infected. Thus, it is difficult to determine which practice with which person actually resulted in HIV transmission.
Nonetheless, the best available estimates indicate that the risk of HIV transmission with receptive anal intercourse is between 1:100 and 1:30, with insertive anal intercourse 1:1000, with receptive vaginal intercourse 1:1000, with insertive vaginal intercourse 1:10,000, and with receptive fellatio with ejaculation 1:1000. The per-contact risk of HIV transmission with other behaviors, including receptive fellatio without ejaculation, insertive fellatio, and cunnilingus, is not known.
All per-contact risk estimates assume that the source is HIV infected. If the HIV status of the source is unknown, the risk of transmission is the risk of transmission multiplied by the probability that the source is HIV infected. This would vary by risk practices, age, and geographic area. A number of cofactors are known to increase the risk of HIV transmission during a given encounter, including the presence of ulcerative or inflammatory sexually transmitted diseases, trauma, menses, and lack of male circumcision.
The risk of acquiring HIV infection from a needlestick with infected blood is approximately 1:300. Factors known to increase the risk of transmission include depth of penetration, hollow bore needles, visible blood on the needle, and advanced stage of disease in the source. The risk of HIV transmission from a mucosal splash with infected blood is unknown but is assumed to be significantly lower.
The risk of acquiring HIV infection from illicit drug use with sharing of needles from an HIV-infected source is estimated to be 1:150. Use of clean needles markedly decreases the chance of HIV transmission, but does not eliminate it if other drug paraphernalia are shared (eg, cookers).
When blood transfusion from an HIV-infected donor occurs, the risk of transmission is 95%. Fortunately, since 1985, blood donor screening has been universally practiced in the United States. Also, persons who have recently engaged in unsafe behaviors (eg, sex with a person at risk for HIV, injection drug use) are not allowed to donate. This essentially eliminates donations from persons who are HIV infected but have not yet developed antibodies (ie, persons in the “window” period). HIV antigen and viral load testing have been added to the screening of blood to further lower the chance of HIV transmission. With these precautions, the chance of HIV transmission with receipt of blood transfusion in the United States is about 1:1,000,000. Between 13% and 40% of children born to HIV-infected mothers contract HIV infection when the mother has not received treatment or when the child has not received perinatal HIV prophylaxis. The risk is higher with vaginal than with cesarean delivery, higher among mothers with high viral loads, and higher among those who breast-feed their children. The combination of prenatal HIV testing and counseling, antiretroviral treatment for infected mothers during pregnancy and for the infant immediately after birth, scheduled cesarean delivery if the mother has a viral load of greater than 1000 copies/mL, and avoidance of breastfeeding has reduced the rate of perinatal transmission of HIV to less than 2% in the United States and Europe.
HIV has not been shown to be transmitted by respiratory droplet spread, by vectors such as mosquitoes, or by casual nonsexual contact. Saliva, sweat, stool, and tears are not considered infectious fluids.
Since the epidemic began in the early 1980s, a total of 1,232,346 Americans have been diagnosed with AIDS. There are an estimated 1.1 million adults and adolescents in the United States living with HIV, 15% of whom are undiagnosed. Young people (age 13 to 24) are the most likely to not know they are infected (51%). In 2016, 39,782 Americans were newly diagnosed with HIV infection. Gay and bisexual men accounted for most of the new diagnoses (67%), and among them, black/African-American men accounted for more new diagnoses (10,223) than Latinos (7425) or whites (7390). Between 2011 and 2015, new HIV diagnoses decreased by 10% among white gay and bisexual males, but increased 4% among African-American gay and bisexual males, and 14% among Latino gay and bisexual men. Heterosexual contacts accounted for 24% (9578) of new HIV diagnoses, women accounted for 19% (7529) and injection drug users for 9% (3425) of new HIV diagnoses, including gay and bisexual men who inject drugs. In 2016, African Americans and Latinos were disproportionately hard hit by the epidemic. Though representing 12% of the US population, African Americans accounted for 44% of new HIV diagnoses (17,528 cases). Latinos, who represent about 18% of the US population, accounted for 25% of new HIV diagnoses (9766 cases).
In general, the progression of HIV-related illness is similar in men and women. However, there are some important differences. Women are at risk for gynecologic complications of HIV, including recurrent candidal vaginitis, pelvic inflammatory disease, and cervical dysplasia and carcinoma. Violence directed against women, pregnancy, and frequent occurrence of drug use and poverty all complicate the treatment of HIV-infected women.
Worldwide there are an estimated 36.9 million persons infected with HIV, with heterosexual spread being the most common mode of transmission for men and women. The reason for the greater risk for transmission with heterosexual intercourse in Africa and Asia than in the United States may relate to cofactors such as general health status, the presence of genital ulcers, relative lack of male circumcision, the number of sexual partners, and different HIV serotypes. It is estimated that in 2016, 19.5 million persons infected with HIV were receiving treatment (53%), up from 17.1 million in 2015.
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