2.3 Animal Vaccination and Sample Collection
Twenty BALB/c mice were randomly divided into four groups with five mice in each group. Mice were intramuscularly immunized as follows. Group 1 was immunized with PBS. Group 2 was immunized with 5 µg HA formulated with an equal volume of MF59. Group 3 was immunized with 5 µg SARS-CoV-2 Omicron BA.5 S formulated with an equal volume of MF59. Group 4 was immunized with 5 µg HA plus 5 µg SARS-CoV-2 Omicron BA.5 S formulated with an equal volume of MF59. As shown in Figure 1, all mice were immunized three times at two-week intervals. Serum was isolated from the collected blood one week after each immunization, heat-inactivated at 56°C for 30 min, and then divided and frozen at -20°C in the refrigerator for further use.