Vaginal pH neutralization by semen as a cofactor of HIV transmission

OBJECTIVE: To explain the high frequency of heterosexual transmission of AIDS in subSaharan countries, by investigating post-intercourse semen retention in the vagina for local pH neutralization as a possible interference factor with acidic inactivation of HIV virions. METHODS: Two semen markers, prostate-specific antigen and prostatic acid phosphatase, were measured and compared with the pH values in the vaginal washes of 69 women from the Central African Republic. The capacity of semen to raise the vaginal pH was also investigated in vitro. RESULTS: Of 61 non-menstrual specimens, 74% contained at least one semen marker. The specimens with high levels of markers (group I) displayed an almost neutral pH (median 6.1), at variance with the semen-free group II (median: 3.7, P<0.003), and with group III (median: 4.0) corresponding to low or past semen retention. The in vitro study confirmed the high neutralization capacity of semen. CONCLUSIONS: It is expected that post-intercourse neutralization of pH will both favor male-to-female transmission and prevent the acidity-associated loss of infectivity of the female-derived virions, thus allowing female-to-male transmission during further sexual contact.