US researchers said Wednesday they have developed a new vaginal gel that might help protect women from HIV even it's applied three hours after sex.

Vaginal gels containing antiretroviral drugs are important for HIV prevention but are all designed to be applied by women before sex, which can interfere with sex practices and limit their use, the researchers wrote in their Science Translational Medicine paper.

They believed a gel that can be applied after sex would be more practical because it will give women more control and would require less preparation before sex and less partner acceptance.

The gel, developed by researchers of the US Centers for Disease Control and Prevention, contains the anti-HIV drug raltegravir.

Using a macaque model to assess efficacy, the researchers found applying the gel 30 minutes before exposure protected two of three macaques from SHIV, a combination of HIV and a related monkey virus.

More importantly, the gel was equally effective when applied after exposure. The researchers said five of six macaques treated with the gel three hours after SHIV exposure remained uninfected after 20 virus challenges and the 10-week follow-up period.

The researchers noted these observations require confirmation in larger numbers of animals.

"In conclusion, we show preclinical in vivo data that support the use of topical integrase inhibitors for HIV prevention," they said.

"This study supports evaluation of this class of drug for HIV prevention ... (and) further evaluation of postcoital modalities in women for enhanced acceptability and compliance."