Why did the HIV vaccine strategy tested in the RV 144 trial use two vaccines in succession?

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sexually transmitted disease Scanning electron micrograph of the spirochete *Treponema pallidum* attached to testicular cell membranes.

The RV 144 trial utilized a "prime-boost" vaccination strategy, which involves administering two different vaccines in succession to enhance the body's immune response. In this approach, the first vaccine (the "prime") is used to initiate an immune response, while the second vaccine (the "boost") is intended to strengthen or broaden that response.1

Each vaccine in the RV 144 trial was specifically designed to target strains of HIV circulating in Southeast Asia. While the trial, which involved over 16,000 volunteers in Thailand, suggested a 31.2 percent reduction in the risk of HIV infection, the results remained controversial due to variations in the statistical significance of the findings across different analyses.1