Why might it be more difficult to prepare a vaccine against noroviruses than against rotaviruses?

Designing efficacious norovirus vaccines remains a major challenge due to several factors:

(1) the extreme genetic variability within the norovirus family and within genogroups

(2) the rapid evolution of antigenically dissimilar pandemic GII. 4 norovirus strains

(3) the lack of lasting immunity upon natural exposure to noroviruses in at least a proportion of the population.