Fig. 3

The neutralization antibody titer against Omicron before and after booster administration. a Schematic illustration of the booster immunization procedure. Half of the guinea pigs primed with each variant were boosted with a Beta-based immunogen, the other half was boosted with Omicron. b–h The neutralization activity of each serum against Omicron pseudovirus was compared with the homologous SARS-CoV-2 variant that was used as immunogen. The neutralization activity of sera sampled at 14 days after the third dose, 90 days after the third dose, and 14 days after the fourth dose (Beta/Omicron booster) was compared. Each dot represents the mean NT₅₀ of three repeated experiments with one guinea pig serum. The fold changes are indicated next to the dots. Data from sera immunized with 3 doses of D614G (b), Alpha (c), Beta (d), Gamma (e), Delta (f), Lambda (g), and Mu (h). Instead of 90 days, guinea pigs were given the booster dose 28 days after the 3rd dose. Student’s t test was used to compare the group immunized with the spike protein of the corresponding variant with Omicron