Mounting evidence shows that male circumcision dramatically reduces the risk of HIV infection, at least for heterosexual males.

According to researchers writing in a 2009 issue of AIDS Patient Care and STDs, the prophylactic effect of male circumcision is owed to the following physical facts:

There are high densities of HIV target cells in the inner mucosal surface of the human foreskin … These HIV target cells lie beneath a protective layer of keratin, which is absent on the inner surface of the prepuce. By removing all or part of the foreskin, circumcision reduces both the number and susceptibility of target cells to HIV infection.

Since 2007, several randomized clinical trials have established that male circumcision could lower the risk of HIV acquisition in heterosexual men by as much as 62 percent. Sixty-two percent! So far, these studies have been limited to African populations that have been particularly hard-hit by AIDS-related casualties. In South Africa, a third of reproductive-aged women are infected. If you’re a 15-year-old living in that country today, there’s a 59 percent chance that you’ll die before reaching your 60th birthday; just 10 years ago, these odds were only 29 percent.

Here’s how the clinical trials basically worked. Thousands of adult, HIV-negative, sexually active, uncircumcised men in Kenya, Uganda and South Africa agreed to be randomly assigned to a circumcision group or a no-circumcision group. Those randomly assigned to the circumcision group had their foreskins removed by medical professionals, were told to abstain from intercourse until their wounds healed (about three weeks—there may actually be a greater risk of HIV infection during this period, so this is vital), and then were instructed to return to the clinic at six-month intervals to test for the virus. The results were unequivocal: two years later, the circumcised males were significantly less likely than their uncircumcised peers to have contracted HIV. In fact, the researchers decided to end these clinical trials early for ethical reasons: with data so clearly showing the advantages of circumcision in an environment rife with the virus, it’s hard to justify a further wait-and-see approach for those men that had been randomly assigned to the no-circumcision group.

For the Ugandan study, 22 of 2,387 circumcised men acquired HIV over the two-year period compared to 45 of 2,430 uncircumcised men who were infected during this time span. Extensive interview methods confirmed that the two groups did not differ in terms of their actual sexual behaviors, enabling the authors to conclude that the results were owed directly to the circumcision intervention. (For those data heads among you, P < 0.00001.) These numbers may not sound massive, but note that they refer to only a 24-month period; over a lifetime, they would become dramatic.

In fact, using the results from the South African study, one group of computer modellerscrunched these numbers to find out how many lives mass neonatal male circumcision could potentially save in this region over a 10-year period. They concluded that male circumcision could save the lives of 300,000 people in Southern Africa alone. Move forward 20 years, they pointed out, and 2 to 7 million deaths could be averted.

It’s presently unknown whether homosexual males would also benefit from circumcision. The studies simply haven’t been done. But Beijing STD specialist Yuhua Ruan and colleagues suspect that circumcision would protect insertive partners (“tops”) against HIV much more than it would receptive partners (“bottoms”). This is because the anal mucosa is highly susceptible to trauma and so the risk of HIV infection through receptive anal sex is very high. The lesser benefit served by being on the receiving end might also apply to heterosexual couples, however. A study published in The Lancet last year found that circumcision in HIV-infected men from Uganda appeared to offer no protection against the virus to their female partners. Thus, although it’s too soon to tell, the real benefits of circumcision may be reserved primarily for heterosexual men or insertive gay men. But that’s still a lot of people whose foreskins may be compromising their health.

In a 2007 report in The Lancet, UCLA infectious disease specialist Sharif Sawires and his colleagues put it bluntly:

In regions where high HIV prevalence exposes the population to risks that have a devastating effect on entire societies, the risks associated with male circumcision could be outweighed by the potential lives saved …

We encourage multicultural, bilateral, and government agencies, along with non-governmental organizations to make this life-saving strategy affordable and safely available to relevant populations bearing the heaviest burden.

These authors certainly aren’t alone in endorsing male circumcision on HIV-preventative grounds. It is now being recommended as a crucial, relatively simple tool against the threat of AIDS by the World Health Organization and UNAIDS. Importantly, of course, these experts also hastily point out that circumcision is just one effective strategy and must be used in conjunction with other preventative measures such as condoms and education.