About 3-7 days after each vaccine, about 4 tablespoons of your blood will be drawn to measure the immune reaction to the vaccine. At that time you will also have a skin test performed to look for the presence or absence of an immune reaction to the vaccine. This will be done with a small injection under the skin on your forearm. About 48 hours later, researchers will measure the area around the injection for redness, which would be a sign that your immune system is reacting to the vaccine.