WASHINGTON (AP) -- Scientists are planning a major study of an experimental HIV vaccine in South Africa later this year.

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The shots are based on the only attempted HIV vaccine ever to show even marginal effectiveness, in Thailand in 2009. The U.S. National Institutes of Health said Wednesday its study in South Africa will use shots modified for better protection.

Planned to begin in November, 5,400 adult volunteers would receive five injections over a year, either vaccine or dummy shots. Results are expected in 2020.

In the 2009 Thai study, a two-vaccine combination cut the risk of HIV infection by 31 percent over 3½ years, but it was more effective the first year. NIH said a small safety trial in South Africa found the new modified shots promising enough for expanded study.