Immunity to coronaviruses does tend to fall within a year or so, but this is believed due to immune memory rather than viral mutation (coronaviruses have a modest mutation rate). This is a different mechanism than influenza. Subsequent reinfections are believed to be mild. A vaccine would provoke a longer immune memory by way of an adjuvant. Discussion of this in the TWIV podcast: http://www.microbe.tv/twiv/twiv-591/