- The first vaccine arm contains the two diphtheria protein containing vaccines tetanus and diphtheria (Td) and pneumococcal conjugate 13 (PCV13) and polio. The second vaccine arm contains the Hepatitis A and Hepatitis B vaccines. Subjects will either get tetanus, diphtheria, polio, and pneumonia vaccines before IL-7 therapy (Sequence 1) or hepatitis A and hepatitis B vaccines before IL-7 therapy (Sequence 2). The response to vaccines will be evaluated 4 weeks after vaccination. This will be followed by IL-7 therapy, then administration of the other group of vaccines. Therefore, subjects on both arms will receive the same set of vaccines, just at different times with respect to IL-7 therapy.