24 out of 31 Massachusetts genotype IBVs are reisolates of H120 vaccine strain.
Only 2 out of 31 are of M41-like pathogenic type.
5 of the Massachusetts genotypes are originated from recombination events between H120-like and other types of IBVs.
Recombination resulted in the emergence of a novel serotype of IBV, although they are Massachusetts genotype by S1 gene analysis.