Context: The BCG vaccine has limitations, and research to develop new TB vaccines is ongoing. A number of potential candidates are currently in phase I and II clinical trials. Two main approaches are being used to attempt to improve the efficacy of available vaccines. One approach involves adding a subunit vaccine to BCG, while the other strategy is attempting to create new and better live vaccines. MVA85A, an example of a subunit vaccine, currently in trials in South Africa, is based on a genetically modified vaccinia virus. Vaccines are hoped to play a significant role in treatment of both latent and active disease.

Question: What vaccine are researchers trying to improve on?
Answer: BCG

Question: What phase of clinical trials have some new vaccine options reached?
Answer: I and II

Question: What does one option propose adding to the existing BCG vaccine? 
Answer: subunit vaccine

Question: What's the name of the subunit vaccine being studied in South Africa?
Answer: MVA85A

Question: What genetically modified virus was used to develop MVA85A?
Answer: vaccinia

Question: What candidate is in phase I and II?
Answer: Unanswerable

Question: What is added to a subunit vaccine?
Answer: Unanswerable

Question: Where is BCG in trials?
Answer: Unanswerable

Question: How many main limitations does the BCG vaccine have?
Answer: Unanswerable

Question: What virus is BCG based on?
Answer: Unanswerable