Presumably as it goes through a community so it finds the local Typhoid Marys and they push the R up. Once all the Typhoid Marys are mopped up, the infection rate then resettles back to normal. Typhoid Mary refused to follow prescribed practice and thus spread typhoid due to her poor hygiene. So part of 'the peak' must be those super spreaders in high risk face to face jobs doing their bit like Cummings to spread the disease around. Upon reflection, Johnson fits this profile (I visited a hospital with COVID patients and shook hands with everybody).