Background: Drugs given to treat cancer (chemotherapy) can weaken the human immune system. But it can also become weaker because of aging. Interleukin (IL)-7, a molecule produced naturally in the body, can help improve the function of the immune system. Researchers want to study the effects of IL-7 on immune system function in two different groups of older people. One group will be people who have received vaccines before IL-7. The other group will be people who have received Vaccines after IL-7.