Taking drug cocktails for four weeks seems to greatly reduce the risk of becoming infected with the virus, which is transmitted through sexual relations, drug use and shared needles, the Centres for Disease Control and Prevention (CDCP) said on Thursday.

These drug cocktails, called highly active antiretroviral therapy (Haart), are routinely taken for life by HIV-infected patients who can afford it and have access. Haart can keep a patient healthy despite infection with the deadly and incurable virus.

The CDCP said there was no ethical way to do a random trial comparing post-exposure prevention to a placebo or dummy pill.

But trials on animals and studies of rape victims and of people at high risk of HIV infection because of their behaviour have shown that taking a two or three-drug cocktail after the possible exposure does prevent infection.

"A 28-day course of Haart is recommended for persons who have had non-occupational exposure to blood, genital secretions, or other potentially infected body fluids of a person known to be HIV infected when that exposure represents a substantial risk for HIV transmission," the CDCP said.

The quicker, the better, it added.

Facts and figures

In its report, the CDC pointed to a study of needlestick injuries to health-care workers. "In this study, the prompt initiation of zidovudine (AZT) was associated with an 81% decrease in the risk for acquiring HIV."

In another trial of 200 homo and bisexual Brazilians at high risk of HIV infection, doctors gave out "starter packs" of AZT and another AIDS drug called lamivudine.

In the group that used the drugs after having unprotected sex, one person became infected, while 11 people in the group that did not take the drugs became infected.

South African rape victims got a similar treatment and none of the women who started the drugs within 48 to 72 hours became infected.

"Although 40,000 new HIV infections occur in the US each year, relatively few exposed persons seek care after non-occupational exposure," the CDCP concluded.