Circumcision removes the foreskin of the penis which mainly acts as a cover to the penis. Normally removing the foreskin makes more vulnerable to any STD, so how could circumcision reduce the risk of a HIV infection?
This is an interesting question, which has not been answered yet. It is also questionable, if this protective effect is present at all. There are around 40 studies on the topic available which have subsequently undergone meta-analysis. In a meta-analysis all data from recent studies which meet certain quality criteria are analysed together. This gives a much bigger dataset and stronger statistical data. It also helps eliminating the effects of too small test groups.
One of these meta-analysis identified circumcision to have a positive effect on the prevention of HIV infections (reference 1). The authors recommended considering it as a preventive measure.
Two other meta-analysis were more cautious on the topic and either found only a weak link (reference 2) or that only "randomized control studies" (reference 3) would provide clear data. The problem with all these studies is, that they are observational studies, where unknown other effects can play a bigger role than the supposed main effect. It is ethically very difficult to make controlled trials on the transmission of deadly diseases in humans. This is also the reason why all trials on this have been stopped.
There is a hypothesis on how circumcision may help preventing the infection with HIV (when you not get infected in the operation itself due to improper hygienic standards). The skin in the penis contains a large number of immune cells (dendritic cells, CD4/CD8-T-cells) which are the target cells of the HIV virus. It is thought that the virus can more easily penetrate the relatively thin skin of the foreskin. It is also possible, that the surface size is important here, as there are reports which show that men with a larger foreskin also have a larger risk of getting infected. See reference 4.
Another hypothesis has to do with the with the bacteria which live under the foreskin. The number of different strains present is much lower in circumcised men, which led to the conclusion that the dendritic cells, which present these to the immune system have an easier job in the circumcised men and are more likely to prevent the infection with HIV. See reference 5 and 6 for more information.
- Male circumcision and risk of HIV infection in sub-Saharan Africa: a systematic review and meta-analysis
- Male circumcision for prevention of heterosexual acquisition of HIV in men
- HIV and male circumcision--a systematic review with assessment of the quality of studies.
- Male circumcision for HIV prevention: current research and programmatic issues.
- Why Circumcision Lowers Risk of HIV
- Male Circumcision Significantly Reduces Prevalence and Load of Genital Anaerobic Bacteria
Unfortunately overlaying all of this is a social aspect that is difficult to eliminate. Virtually any male researcher into this topic has a vested interest in vindicating his own penile condition. Men feel strongly about the perfection of their own penises, however they find them.
It is very clear from the discussion and tone of many of these studies that the researchers are themselves circumcised, and want circumcision to be treated as the norm. The same effect is possible in the other direction, but seems to occur to a lesser extent because intactness is the biological norm and requires no action to make it so.
For that reason, studies out of the USA (including those led by American researchers in Africa) disproportionately favour circumcision, while those out of Europe do not. One reason this bias can creep in is that it is difficult to do double-blinded and/or placebo-controlled trials of circumcision. Thus experimenter and experimentee effects can creep in at every stage.
Circumcision is an intervention with deep social, religious, historical and cultural roots, and once it has become customary, rationality about it is difficult to achieve. Especially considering the operation on healthy infants, the fact that the foreskin is an integral part of an adult sexual organ gets lost from sight.
It is very common for the circumcised penis to be treated as the norm (with terms like "uncircumcised" common, where "normal" "whole" or "intact" would apply to any other body part that had not been surgically reduced). After that, circumcision may be treated as equal and opposite to the alternative of doing nothing, when it is no such thing. Thus the yawning gap between "circumcision [somewhat] reduces the incidence of X disease" and "all male babies should be circumcised" can be leapt in a single bound.