I touched on this in an earlier answer about the historical accuracy of part of Follett's World Without End:

Additionally, news of the great mortality traveled ahead of the pandemic itself. The Bishop of Rochester worried himself half to death about the oncoming plague during winter 1348, even though it didn't show up in Kent until the next spring. English archbishops in particular were quite active at arranging penitential processions and organizing local monasteries to say Masses and special prayers in attempt to prevent pestilence from decimating their dioceses.

Penitential processions and extra Masses worked approximately as well as prophylaxis as you might think. Actually, the situation was so desperate by the time pestilence was on the ground that a major school of thought developed that people shouldn't even try medical treatment of the plague; they should just accept it as punishment for collective sins. Just do more and more penance, be more and more contrite.