A vaccine might prevent infection completely by triggering a strong antibody response
A vaccine might trigger a strong T-cell response to combat an infection and clear it from the body to prevent disease. This is the way we are able to fight back against a cold or the flu.
A third approach is to slow down the infection, making the time from getting HIV to developing AIDS stretch out even longer by helping to control viral load. Also, people with a lower viral load are less likely to transmit their infection to others.
A vaccine may also benefit the wider public, even those who are not vaccinated. This idea is called community immunity or herd immunity: if enough people get vaccinated, making those people less likely to transmit HIV to others, then the wider community will have some protection as a result. This can help protect people who are unable to be vaccinated, such as children or people with other medical conditions.
Perhaps one vaccine will not be enough to trigger the right combination of immune responses, and we will need a combination of products to attack the virus, similar to combination therapy used to treat people who are living wiht HIV.
In this scenario, one vaccine is given to get the immune system ready for action, or “prime” it. Priming may require more than one dose. One or more different vaccines are given at the same time or at later time points to “boost” the immune response. Several studies currently underway are looking at various prime-boost combinations.