highly significantp ≤ 0.001
Tests of homogeneity for the stratifying variables time since last vaccination, grade at first vaccination, year of first vaccination and season at first vaccination were all highly significant (p ≤ 0.001, Table 2 ).
Tests of homogeneity for the stratifying variables time since last vaccination, grade at first vaccination, year of first vaccination and season at first vaccination were all highly significant (p ≤ 0.001, Table 2 ).