The BCG vaccine is available to immunize people against tuberculosis. Had Lance been vaccinated, he would not have been concerned about becoming infected on the airplane. Why was he NOT vaccinated?

An immunized person can show false positive results during the test.

Explanation:

BCG (bacille Calmette-Guerin) is the vaccine which is given for tuberculosis. This vaccine is used in many countries in which the chances of spreading Tb is high in children which causes meningitis and miliary disease in children.  

In United States, BCG is not recommended because the risk here is very low and the vaccinated person can show false-positive results for the disease which will result in hindering the work of epidemiologists who are tracking how and from where disease spread.