Problem pathogens and vaccine strategies

Question 1

Why are there so many diseases for which vaccines do not exist?

Answer 1

• “Escape” – development of mutations to avoid the immune system (eg influenza and HIV)
• Evolution of protective mechanisms
  (eg S. pneumoniae and HIV)
• Integration into the host genome (eg herpes simplex)
• Dormancy in “immune privileged” sites
  (eg M. tuberculosis and Ebola)
• Strain variation extending beyond immunological memory(e.g. dengue)

Question 2

What’s so difficult about HIV?

Answer 2

1. HIV attacks white blood cells of the immune system
   In HIV infection, the number of white blood cells goes down and the patient becomes immunodeficient
2. There are only a limited number of targets on the HIV surface to attack and they are well hidden.
3. HIV releases envelope as a decoy.
4. HIV has a very high mutation rate. It is a moving target.
5. The aim of an HIV vaccine has to be to prevent
   infection, not to respond to infection. An HIV vaccine needs to stop the virus penetrating the outer layer (epithelium) of the skin – vagina
   or rectum