Abstract
<jats:p>Background Improved understanding of how variants cause breakthrough infection, despite pre-existing immunity, is needed to advance development of next-generation SARS-CoV-2 vaccines, including those that may provide cross-variant protection or block transmission. SARS-CoV-2 controlled human infection models (CHIMs) may therefore identify correlates of protection and accelerate the development of new interventions. Methods Healthy vaccinated adults aged 18-30 years were inoculated intranasally in a stepwise dose-escalation CHIM with doses from 1x102 TCID50 to 1x106 TCID50 of SARS-CoV-2 Delta variant. Within the 1x106 TCID50 group, participants were selected for serum neutralising antibody titres (NT50) less than or equal to 1:80. Post-inoculation, participants were quarantined for up to 14 days. Outpatient follow-up continued for 12 months. The primary aim was to elicit safe, well-tolerated Delta SARS-CoV-2 breakthrough infection at a rate of over 50%. Findings Forty-six participants were inoculated; 22 during dose-escalation with no resultant infections, and 24 at the highest dose, of whom 18 were screened for low serum neutralising antibodies. Sustained infection with mild-to-moderate symptoms occurred in 33% (6/18) of the sero-selected group, with highly variable viral loads, viral emissions and symptoms. Serum neutralising antibody, anti-N IgG and to a lesser extent, mucosal anti-S IgA and N-specific T cells most strongly predicted protection from virologically-defined infection. Higher neutralisation, serum and nasal anti-N IgG level, and N-specific T cell responses correlated with lower viral load, while baseline nasal anti-S IgA was associated with lower symptom scores. Transient infection was additionally observed in 6 participants and was associated with higher baseline N-specific T cell responses than those that developed sustained infection. Interpretation Susceptibility to SARS-CoV-2 breakthrough infection in those with hybrid immunity is strongly associated with low levels of pre-existing antibody, but the diversity of immune markers associated with protection implies that additional benefits may be conferred by multi-pronged immunity.</jats:p>