People with HIV can reduce the risk of infecting their sex partners by more than 90 percent if they start treatment with antiretroviral drugs when their immune system is still relatively healthy, researchers announced Thursday. The study, which included 1,763 mostly heterosexual couples from nine countries, was supposed to last until 2015, but the results were released early because of the significance of the findings. The research confirmed a belief held by many scientists and physicians — that starting drug therapy early can help to limit rates of transmission of the virus that causes AIDS.