WASHINGTON — Treating HIV right away, before patients are too sick, dramatically lowers their chances of spreading the AIDS virus to a sexual partner, says a major international study that may convince more doctors to offer medication sooner.

The nine-nation study offers convincing evidence of what scientists have long believed — that HIV medicines don’t just benefit the patient, but may act as a preventive by making those people less infectious. Earlier treatment in the study meant patients were 96 percent less likely to spread the virus to their uninfected partners, according to preliminary results announced Thursday by the U.S. National Institutes of Health, which oversaw the research.

Those findings were striking enough that the NIH said it was stopping the study four years ahead of schedule to get the word out.

The study included couples from Botswana, Brazil, India, Kenya, Malawi, South Africa, Thailand, Zimbabwe, as well as a few from the United States.

When HIV patients should start taking antiviral drugs is an important question. The pills are lifesaving but also expensive — up to $15,000 a year in the U.S. — and carry a range of side effects from diarrhea to liver damage.

NIH’s Dr. Anthony Fauci said the new study promises to change practice worldwide. In developing countries, where the drugs cost a few hundred dollars a year, patients tend to be far sicker before getting medication. Even in the U.S., where therapy starts sooner, doctors do not always treat as early as was done in this study.

“It has less to do with a decision about what’s good for you from a personal health standpoint than what is the extra added benefit from starting earlier, i.e., transmission, especially if you have a partner who’s uninfected,” said Fauci, director of the National Institute of Allergy and Infectious Diseases.

Condoms remain crucial for protection — the medications do not change that longtime recommendation. All 1,763 couples in the study, where one partner had HIV and the other didn’t, were urged to use them.

“HIV-positive people cannot assume they are not infectious simply because they are already on treatment medications,” warned Dr. Thomas Frieden, director of the Centers for Disease Control and Prevention.

Previous research has suggested that HIV patients who use the medications are less likely to spread infection. But the $73 million study announced Thursday is the first to rigorously test that.

The couples, most of whom were heterosexual, were randomly divided. Among half, the HIV-infected partner started medication immediately after diagnosis. Among the other half, the infected partner delayed using the drugs until his or her level of CD4 cells — a way to measure the strength of the immune system — dropped below 250 or symptoms appeared.

In 28 couples, the uninfected person became infected by their partner. Only one of those infections occurred among the couples where the infected person was treated early, Fauci said.

The other 27 cases in which HIV spread involved couples that delayed drug treatment.

Importantly, more than half of those infections occurred when the patient’s CD4 count remained greater than 350, Fauci noted. That number indicates only moderate immune damage. Most developing countries don’t offer treatment until CD4 levels dip lower than that.

U.S. guidelines recently were changed to recommend that treatment begin when that immune system number is below 500, although many doctors haven’t yet begun following that advice, said Dr. Michael Horberg of the HIV Medicine Association and HIV-AIDS director for Kaiser Permanente. Some experts would treat even sooner.

The earlier treatment also helped reduce some complications — such as a form of tuberculosis — in the original patients, but there was no significant difference in deaths between the two groups.