Post-mRNA-1273 boost Bab titers were greater in participants who had a previous SARS-CoV-2 infection after the CoronaVac primary vaccination (i.e., virus breakthrough), and when a longer period of time (> 4 months) had elapsed since the most recent prior spike antigen exposure (either the second CoronaVac dose or a breakthrough infection) compared with more recent exposure. Here, CHMP5 is linked to infection.