The surprising failure of a large clinical trial of H.I.V.-prevention methods in Africa — and the elaborate deceptions employed by the women in it — have opened an ethical debate about how to run such studies in poor countries and have already changed the design of some that are now underway.

Scientists who conduct clinical trials are now testing participants’ blood more often and holding group discussions to quell rumors and urge participants to take their medications diligently.

As a result of the failed trial, scientists are arguing vigorously about the extent to which it is ethical to pay participants for their time, and whether results of trials that do so can be trusted.

The trial — known by the acronym Voice, for Vaginal and Oral Interventions to Control the Epidemic — was abruptly halted by independent safety monitors because it was not working: Women who were given pills or vaginal gels containing anti-H.I.V. drugs were becoming infected at roughly the same rate as women who were given placebos.