Case reports of oral transmission of HIV are rare. The second kind of research study begins by recruiting a large group of participants, called a cohort, and studying them over a long period of time. Members of a cohort are called in at least once a year to take an HIV antibody test and are interviewed in detail about their sexual activities. Over time, researchers use the data to associate specific sexual behaviors with their relative risk of HIV transmission. It was this method that established, early on in the AIDS epidemic, the extreme danger of anal intercourse for HIV transmission.
Large cohorts yield more reliable statistics than case reports.
A total of 746 homosexual men were studied every six months. These factors are considered to give a very accurate and unbiased picture of the epidemic compared to any other study of AIDS that has been attempted. The study group was looking for evidence of oral transmission of HIV but didn't find any. Investigators found 21 HIV-negative cohort members who had no receptive anal sex or fisting. After almost two years of following this subgroup, only one man had become HIV-positive.