SARS-CoV-2 fusion-peptide-directed antibodies are elicited by natural infection and can mediate broad sarbecovirus neutralization

Studies have demonstrated that repeated mRNA vaccination enhances the breadth of neutralization against diverse severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants. However, the development of antibodies capable of neutralizing across the Coronavirinae subfamily is poorly understood. In this study, we analyze serum samples to determine their neutralization breadth and potency and identify their antigenic targets. Using a cohort of older individuals and healthcare workers, we track correlates of broad neutralizing responses, including fusion peptide (FP) antibody elicitation. We find that although broadly neutralizing responses are often a result of receptor-binding domain (RBD)-specific antibodies, a rare subset of donors produce FP-specific broadly neutralizing responses. Interestingly, FP-specific antibodies are not observed in COVID-naive individuals, irrespective of vaccination regimen; rather, they occur following natural infection or vaccine breakthrough. This study highlights which epitope targets underpin broadly neutralizing antibody responses to coronaviruses and suggests that existing vaccines are insufficient to promote the elicitation of FP-directed broadly neutralizing coronavirus antibodies.