HIV-1 can be transmitted through vaginal, anal, and oral sex, whether it’s with a same-sex or different-sex partner.
But the risk of developing it seems to vary depending on how the disease is transmitted.
In fact, the HIV virus may be more virulent when passed between male and female partners who have penile-vaginal sex than among men who have anal sex with other men, according to a new study from the Indian Institute of Science.
Researchers say the reason may be that different strains of the HIV-1 virus are transmitted among these distinct populations, and the subtype most commonly transmitted among people who have penis-vagina sex may simply be more virulent.
The researchers studied CD4 counts among 340,000 individuals with HIV in dozens of countries worldwide.
CD4 cells, also known as T-cells or white blood cells, are part of the immune system and are activated in response to infections. With HIV, the virus attacks the immune system and a low initial CD4 cell count is considered a sign of a more severe infection.
The researchers found that HIV-1 cases transmitted via penile-vaginal intercourse correlated with lower CD4 counts than cases among men who had sex with other men (MSM) — a group that more typically engages in anal intercourse.
In the past,
However, in this study, researchers said “remarkably, we found that HET [heterosexual individuals] consistently had lower early CD4 counts than MSM.”
The researchers speculated that distinctive strains of HIV-1 circulating in these populations may vary in their ability to overcome “
“Different modes of transmission of HIV from one individual to another can exert different bottlenecks on the virus,” wrote Narenda Dixit, PhD, a professor of chemical engineering at the Indian Institute and a lead study author.
He added that the study “reveals more virulent transmitted HIV strains in heterosexual individuals than men who have sex with men, potentially affecting the HIV epidemic differently in these groups.”
The study suggested that different strains of HIV may circulate among different populations, but the authors weren’t able to identify those strains.
Shyam Kottilil, PhD, a professor of medicine at the Institute of Human Virology at the University of Maryland School of Medicine, told Healthline that the population size and global scope of the study are impressive, but that the study has some limitations.
In the United States, for example, most HIV cases are with the B subtype, or “clade,” but there are at least nine different HIV clades circulating worldwide.
“You can’t control for that,” said Kottilil.
While the study was not able to isolate the infection strain on a case-by-case basis, the authors noted that other studies have shown that 90 percent of MSM had subtype B infections, compared to just 10 percent of heterosexual individuals.
This lends support to the argument that there is little crossover between the two populations and that a tougher strain may be circulating among the heterosexual population.
“Mixing between the two groups would have led to a more similar distribution of subtypes in the two groups,” the study authors wrote. “The two groups thus appear to have remained largely segregated.”
The study also relied on a snapshot of early CD4 cell counts to determine infection virulence, which are known to rise immediately after infection and then drop off.
Kottilil said that levels of immune system cells are “quite variable,” even on a day-to-day basis, which could make them an unreliable marker for the extent of an HIV case.
Past research has indicated that both
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