Fig. 5. Humoral responses after homologous or heterologous prime-boost vaccination.

A C57BL/6 mice received an intramuscular prime immunization with the spike-encoding DNA (10 µg), Ad5-S (10⁷ infectious units), or the mRNA vaccine, Comirnaty^® (1 µg). Mice from the heterologous prime-boost groups were boosted four weeks later intranasally with Ad5-S (10⁷ infectious units). The homologous prime-boost groups received a second dose of mRNA (1 µg) or Ad5-S (10⁷ infectious units) intramuscularly. Serum antibody responses were analysed 21 days and mucosal immune responses four weeks after the boost immunizations. Spike-specific IgG (B) were assessed by a flow cytometric approach (dilutions: Sera 1:800, BAL 1:20). BAL samples were tested for spike-specific IgA directed against RBD by ELISA (C). Plaque reduction neutralization titres (PRNT75) were determined by in vitro neutralization assays (D). Bars represent group medians overlaid with individual data points; sera all groups n = 8; BALs RNA-Ad5 n = 7, other groups n = 8 (out of two independent experiments). Data were analysed by one-way ANOVA followed by Tukey’s post test (B and C) or Kruskal–Wallis test (one-way ANOVA) followed by Dunn’s multiple comparison test (D). Statistically significant differences are indicated only among the different vaccine groups; p values indicate significant differences (*p < 0.05; **p < 0.005; ***p < 0.0005; ****p < 0.0001).