In an unexpected setback for a new form of AIDS prevention, scientists on Monday halted a study in Africa intended to find out whether a daily antiretroviral pill can prevent women from becoming infected with the AIDS virus.

Early data showed no evidence that the pill was working.

Women taking the medication, Truvada, were just as likely as those taking a placebo to become infected, according to an independent panel that analyzed the results after the study had enrolled about half the 4,000 women researchers had hoped to enlist.

Of the 1,900 women taking Truvada or a placebo, 28 in each group had become infected as of last week, according to FHI, formerly Family Health International, the nonprofit group that was conducting the study in South Africa, Kenya and Tanzania.

“It’s surprising, disappointing and frustrating,” said Mitchell Warren, executive director of AVAC, an advocacy group for AIDS prevention in New York. “But there’s a strong rationale for continuing other trials in women in hopes of obtaining better results in the future.”