Level of HIV virus in the blood is major determinant of sexual transmission

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A study of heterosexual couples in several sub-Saharan African countries found that the amount of HIV virus in an infected person’s blood is the major determinant of whether or not they will pass the virus on to their uninfected sexual partners.

Published today in The Journal of Infectious Diseases (JID), the article “Determinants of Per-Coital-Act HIV-1 Infectivity among African HIV-1-Serodiscordant Couples” calculated the risk of HIV transmission from an infected person to an uninfected person per sexual act. 

“Overall infectivity was 1-2 cases per 1000 coital acts, similar to that reported during latency in low-income countries but higher than estimates from many industrialized nations,” states the study’s accompanying commentary “Probability of Heterosexual HIV-1 Transmission per Coital Act in Sub-Saharan Africa,” by Ronald H. Gray and Maria Wawer at Johns Hopkins University Bloomberg School of Public Health. “The HIV load in the index infected partner was the main driver of transmission, with a 2.9-fold adjusted risk of infection per log10 increment in viral load.” A log increment refers to the number of copies of HIV-1 RNA per milliliter of plasma. Simply put, the higher the level of HIV-1 RNA in the infected partner’s plasma, the higher the risk of transmission.

The study authors utilized data from 3,297 HIV-discordant couples (where one is infected with HIV and the other is HIV-negative) enrolled in a randomized trial of acyclovir Herpes Simplex Virus (HSV)-2 suppressive therapy for prevention of HIV-1 transmission. The acyclovir, however, did not affect HIV acquisition. The large cohort had frequent follow up visits over a 24-month period where the HIV-1 RNA blood levels in the HIV-infected partner were measured and genetic testing was performed on the original uninfected partner to determine if the virus was acquired from inside the relationship or from an outside partner. The authors recorded 86 linked HIV-1 transmissions during follow up.

“The findings also showed that the risk of an HIV-infected man transmitting an infection to a woman not infected with HIV was about twice the risk of an HIV-infected woman transmitting to an HIV-uninfected man,” according to a JID press release. That can be attributed to the difference in viral loads between men and women, however, the authors noted.

The couples that agreed to receive voluntary counseling and testing reported using condoms 93 percent of the time – and condom use was 78 percent effective at preventing HIV transmission among the study population.

Other important notes from the study, investigators reported:

  • a 2.14-fold increased risk of infection associated with HSV-2 positivity in the HIV-1 uninfected partner.
  • a 2.65-fold increased risk of infection associated with genital ulcer disease (GUD) by self-report or exam in HIV-1 uninfected partners.
  • circumcision in male HIV-1 uninfected partners was associated with significantly lower infectivity (reported reduction in female-to-male transmission of approximately 47 percent).
  • infectivity declined as the age of the HIV-1–uninfected partner increased.

One noted limitation, the study did not look at early or late-stage infection, which previous studies have shown are both associated with higher infectivity per sexual act.

The authors note the importance of scaling up various key prevention interventions to lower HIV transmission risk, including antiretroviral therapy, voluntary medical male circumcision, condom promotion and treatment of other sexually transmitted infections.

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