Two different approaches can be used to prevent transmission of HIV: behavioral and biomedical. Behavioral approaches are directed at reducing "risky" actions that are known to transmit HIV, and are directed at uninfected people to prevent acquisition of HIV (primary prevention) and at people who are infected to reduce transmission (secondary prevention). Biomedical approaches, some of which remain theoretical rather than realized objectives, can also address acquisition and transmission. Among biomedical approaches, vaccines hold the greatest hope to curtail AIDS worldwide. Other approaches include: post-exposure and pre-exposure prophylaxis with ARVs and microbicides. ARVs have been highly effective in preventing perinatal transmission.