IM WT vaccine-derived antibodies do not suppress the latter mucosal omicron vaccine boost response. (A) Vaccination and serum transfer schedule. Donor mice were vaccinated, and then, sera were collected and transferred to naive recipient mice. Recipient mice were vaccinated with Ad-ο IM or IN. Sera from recipient mice were collected after transfer of sera before vaccination (day 51) and 3 wk after Ad-ο vaccination (day 72). (B) Levels of anti-WT spike IgG and anti-ChAdOx1 IgG in recipient mouse sera, after transfer of donor sera, and before Ad-ο vaccination (day 51). Reference negative control levels of IgG on day 51 in mice that received naive sera were also included in the figure. (C) Levels of anti-omicron spike IgG and o-ACE2^comp-Abs in mouse sera, after transfer of donor sera, and before Ad-o vaccination (day 51). (D) Levels of anti-omicron spike IgG and o-ACE2^comp-Abs in mouse sera, after transfer of donor sera, and after Ad-o vaccination (day 72). Reference groups following Ad-o^IM and Ad-o^IN without serum transfer are included. (E) Absolute change (day 72–day 51) in anti-omicron spike IgG and o-ACE2^comp-Ab levels. For data in C–E, a one-way ANOVA test with post hoc comparison between all groups was performed. For all data, *P < 0.05, **P < 0.01, ***P < 0.001, ****P ≤ 0.0001. On violin plots, the dashed black line represents the group median and dots represent individual mice. Data are representative of two independent experiments, where experiment 1 (shown) was n = 6 per group and experiment 2 (n = 3–6 per group) was n = 6 for test group “WT^IM+WT^IM>o^IM” and n = 3 for test group “WT^IM+WT^IM>o^IN.”