The Urabe AM9 mumps vaccine is a mixture of viruses differing at amino acid 335 of the hemagglutinin–neuraminidase gene with one form associated with disease. Journal of infectious diseases

The reason for the high incidence of vaccine-associated meningitis due to the Urabe AM9 vaccine was assessed by comparing the nucleotide (nt) sequence of the hemagglutinin-neuraminidase (HN) gene from vaccine virus to those of viruses isolated from persons with postvaccination meningitis. A GI 08L...A nt substitution that was predicted to result in a Glu 33S- Lys reversion in the HN protein was detected between Urabe AM9 (G) and postvaccine meningitis mumps virus isolates (A). Further analysis showed that the Urabe AM9 vaccine was a mixture of viruses with wild type (A) and variant (G) nt at position 1081. Urabe AM9 vaccinees who developed meningitis or parotitis possessed predominantly A (98%-100%) at nt 1081, indicating strong selection of the wild type (A) form relative to the variant (G) form. Mumps virus homogeneous for the variant Glu 335 form of the HN gene may be safer than the original Urabe AM9 vaccine. Mumps virus is a Paramyxovirus of the family Paramyxovir-idae, a group of enveloped, nonsegmented, negative-stranded RNA viruses. Infection with mumps virus is established by droplet spread to respiratory mucosa; viremia follows primary replication at this site, leading to infection of many organs