Antibodies from IM WT vaccine priming suppress the latter IM omicron vaccine boost response. (A) Vaccination and serum transfer schedule. Donor mice were vaccinated (or left unvaccinated as a negative control), with sera collected and transferred to naive recipient mice. Recipient mice were vaccinated with Ad-ο IM. Sera from recipient mice were collected after transfer of donor 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-o vaccination (day 51). Reference levels of IgG following an in vivo Ad-WT^IM and Ad-WT^IM+Ad-WT^IM are shown. (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). (E) Absolute change (day 72–day 51) in anti-omicron spike IgG and o-ACE2^comp-Ab levels. (F) Correlation of absolute change (day 72–day 51) in anti-omicron spike IgG and o-ACE2^comp-Ab levels against levels of anti-WT spike IgG transferred (measured on day 51). Data in D were analyzed by one-way ANOVA test on IgG data, and Kruskal–Wallis test on o-ACE2^comp-Abs data. Data in E were analyzed by one-way ANOVA test with post hoc comparison against mice receiving naive sera as the reference. (F) Pearson correlations are shown. For all data, *P < 0.05, **P < 0.01, and ****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 was n = 5 per group.