The majority of global HIV infections are caused by a single group of virus, which is divided into nine different subtypes, or clades, designated by the letters A through K. HIV clades vary in prevalence throughout the world. For example, HIV clade B is found mostly throughout North America and Europe, while the epidemic in South Africa and India is due to HIV clade C. Researchers are trying to develop an AIDS vaccine candidate that offers the broadest possible protection. Trials with vaccines from viruses in several clades are underway. Merck and the HIV Vaccine Trials Network (HVTN) are testing how well a vaccine from one clade can protect against virus from another in a trial in South Africa.
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