Given that the description of the case below, identify whether you would use a randomized block design, a completely randomized design or a matched pairs design.
HIV Vaccine The HIV Trials Network is conducting a study to test the effectiveness of
two different experimental HIV vaccines. Subjects will consist of 80 pairs of twins. For each pair of twins, one of the subjects will be treated with the DNA vaccine and the other twin will be treated with the adenoviral vector vaccine.