are similar to viroids
are only needed once
kill viruses
stimulate an immune response

Stimulate an immune response

Explanation:

The vaccine is a preparation that contains a weakened or killed form of microorganism (or part of them) that stimulates the immune system to produce memory antibodies to recognize that microorganism in the future. After the body produces the memory antibodies, if a person is in contact with that microorganism the body recognized them as “strange” and the body response to killing them is faster before the infection. In some cases is necessary to apply various doses of the vaccine in a certain time to maintain the memory antibodies.

if the hares very faster than they are now, and thus the lynx's hunting success rate drops because it will be harder to catch them, the rate that will change will be the pray-rate. the pray-rate will change because more hares will be able to survive, thus more individuals for reproduction, so the end result will be increasing population of hares.

the predator-rate will remain pretty much the same because there will be more than enough pray, but the hunting success rate is stagnant.