According to the Male Circumcision Consortium newsletter, the researchers followed about 1,500 men who took part in a randomised controlled trial -- a scientific experiment commonly used in testing the effectiveness of healthcare services -- of male circumcision conducted in Kisumu.

That trial, and similar ones in South Africa and Uganda, showed that circumcision reduced chances of infection from a female partner by about 60 per cent.

"In the follow-up study, we found that this protective effect is clearly sustained, and possibly even strengthened, over time," said principal investigator Robert Bailey of the University of Illinois at Chicago.

The three randomised trials were stopped early after 18 to 24 months when it became clear that circumcision offered significant protection against HIV infection, but left open the question whether that protective effect is sustained over time.

Prof Bailey and his colleagues conducted the follow-up study to find out whether male circumcision offered long-term protection in 54 months.