Serum neutralizing antibody titer of vaccinated mice. BalB/C mice were immunized with A) 100 μg monovalent VLPs or virions, B) tetravalent VLPs or virions (25 μg of each serotype) for three times at two weeks interval. At day 42, the serum neutralizing antibodies were assessed using CPE-determination assays. Data of each group was expressed as geometric mean titer (GMT) with an S.D. bar (n = 5). *indicates statistical significance (P < 0.05).