By 1920 the bacteria that caused the plague was well known, as was the transmission cycle (rat/rodent-flea-person). The transmission of the pneumonic form was also well known, an epidemic usually started with the bubonic form and sometimes "became" pneumonic. In any case the reason plague outbreaks were where they were in "modern" times was that you didn't have the sorts of living conditions in most developed countries where folks were likely to get get fleas from rats, there was a much higher standard of hygiene and sanitation. In the trenches, conditions were a different story, in spite of the delousing procedures which were of limited effectiveness, though much better than in the past.
Between sanitation, rat control/flea control, some quarantining you won't see the plague spread anything like the flu. Using the procedures available at the time, flu was containable to some extent and they did not know what caused it (flu virus not identified). Plague outbreaks in India, China, and other spots where conditions were highly favorable to the transmission cycle could have been worse, massive involvement in Europe and North America, not really absent some improbable bacterial mutation.