Consistent condom use reduces the risk of heterosexual HIV transmission by about 80% over the long-term. Where one partner of a couple has HIV infection, consistent condom use results in rates of HIV infection for the uninfected person below 1% per year. Some data support the equivalence of internal condoms to latex condoms, but the evidence is not definitive. A vaginal gel containing tenofovir, a reverse transcriptase inhibitor, when used immediately before sex, was shown to reduce infection rates by roughly 40% among African women. In contrast, Use of the spermicide nonoxynol-9 may increase the risk of transmission because it causes vaginal and rectal irritation. Studies conducted in sub-Saharan Africa have found that circumcision reduces the risk of HIV infection in heterosexual men between 38 and 66% over two years. Based on these studies, the World Health Organization and UNAIDS both recommended male circumcision as a method of preventing female-to-male HIV transmission in 2007. Whether it protects against male-to-female transmission is disputed and whether it is of benefit in developed countries and among men who have sex with men is undetermined. For men who have sex with men, there is some evidence that the penetrative partner has a lower chance of contracting HIV.
Programs encouraging sexual abstinence do not appear to affect subsequent HIV risk in high-income countries. Evidence for a benefit from peer education is equally poor. Comprehensive sexual education provided at school may decrease high risk behavior. A substantial minority of young people continue to engage in high-risk practices despite HIV/AIDS knowledge, underestimating their own risk of becoming infected with HIV.
Early treatment of HIV-infected people with antiretrovirals protected 96% of partners from infection. Pre-exposure prophylaxis with a daily dose of tenofovir with or without emtricitabine is effective in a number of groups, including men who have sex with men, couples where one is HIV positive, and young heterosexuals in Africa.
Universal precautions within the health-care environment are believed to be effective in decreasing the risk of HIV. Intravenous drug use is an important risk factor and harm reduction strategies such as needle-exchange programs and opioid substitution therapy appear effective in decreasing this risk.
A course of antiretrovirals administered within 48 to 72 hours after exposure to HIV-positive blood or genital secretions is referred to as post-exposure prophylaxis. The use of the single agent zidovudine reduces the risk of subsequent HIV infection fivefold following a needle stick injury. Treatment is recommended after sexual assault when the perpetrators are known to be HIV positive, but is controversial when their HIV status is unknown. Current treatment regimens typically use lopinavir/ ritonavir and lamivudine/zidovudine or emtricitabine/ tenofovir and may decrease the risk further. The duration of treatment is usually four weeks, and is associated with significant rates of adverse effects (for zidovudine about 70% including: nausea 24%, fatigue 22%, emotional distress 13%, and headaches 9%).
Programs to prevent the transmission of HIV from mothers to children can reduce rates of transmission by 92-99%. This primarily involves the use of a combination of antivirals during pregnancy and after birth in the infant but also potentially include bottle feeding rather than breastfeeding. If replacement feeding is acceptable, feasible, affordable, sustainable and safe mothers should avoid breast-feeding their infants; however, exclusive breast-feeding is recommended during the first months of life if this is not the case. If exclusive breast feeding is carried out the provision of extended antiretroviral prophylaxis to the infant decreases the risk of transmission.
As of 2012, no effective vaccine for HIV or AIDS is known. A single trial of the vaccine RV 144 found a partial efficacy rate around 30% and has stimulated optimism in the research community regarding developing a truly effective vaccine. Further trials of the vaccine are ongoing.