We have found it somewhat difficult to add vaccines to the game in a way that is both realistic and simple.
The game can be started either with the vaccine card in the box of component causes (simulates a disease like seasonal flu for which the vaccine is administered before the flu season starts) or by waiting until a few cases of disease occur before adding the vaccine to the box of component causes (simulates a new or emerging disease for which a vaccine might need to be developed).
Players who receive the vaccine can write "VACCINE" on their card with arrows pointing to which sufficient cause completions were "blocked" by the vaccine. We have had some discussion about the scenarios in which blocking would occur; for example, is the vaccine only effective if it's obtained before the initial exposure to the virus or is it still effective if the player has been exposed to the virus but does not yet have the disease (no pie has been completed)? This might vary by type of virus and the instructor can choose the scenario that best fits the game.
When vaccines are introduced and transmission of the virus is allowed, herd immunity can often be demonstrated when a player who is not vaccinated is surrounded by vaccinated players and therefore does not get the disease (unless the virus card is pulled from the box and the player has all other component causes to complete the pie).