CD4 and infection: The model took into account participant age, gender, number of prior COVID-19 vaccinations, number of self-reported prior infections, days since last antigen exposure (either vaccination or infection), baseline IgG titres against Ancestral or BA.1 spike, baseline nAb titres against ancestral, BA.1 or XBB virus, baseline frequencies of Ancestral, BA.1 or cross-reactive memory B cells, and baseline frequencies of AIM+ CD4+ cTFH or Tmem.