GENEVA, Feb 1 (Reuters) - Anti-retroviral drug treatments can dramatically reduce the level of HIV virus in the blood but transmission risks remain, United Nations health agencies said on Friday.

The World Health Organisation (WHO) and UNAIDS, responding to a study published by Switzerland's Federal AIDS Commission, said "correct and consistent use of condoms" was the best way to prevent the spread of the AIDS virus between sexual partners.

People taking anti-retrovirals can have undetectable amounts of HIV virus in their blood "at certain stages of their treatment", the Geneva-based agencies said in a statement.

"However, it has not been proven to completely eliminate the risk of transmitting the virus," UNAIDS and WHO said.

"More research is needed to determine the degree to which the viral load in blood predicts the risk of HIV transmission and to determine the association between the viral load in blood and viral load in semen and vaginal secretions."

They also stressed that other sexually transmitted diseases may contribute to transmission rates, further underscoring the need for "a comprehensive HIV prevention package" that includes mutual fidelity and a reduced number of sexual partners. (Reporting by Laura MacInnis; Editing by Janet Lawrence)

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