Biomarkers of vaccine safety and efficacy in vulnerable populations: Lessons from the fourth international precision vaccines conference.

Vaccination has been a cornerstone of public health, substantially reducing the global burden of infectious diseases, notably evident during the COVID-19 pandemic caused by SARS-CoV-2. However, vulnerable populations (VPs), including those in extreme age groups and those with underlying health conditions, have borne a disproportionate burden of morbidity and mortality from infectious diseases. Understanding vaccine immunogenicity in these populations is crucial for developing effective vaccines. Characterizing vaccine responses in VPs presents unique challenges due to under-vaccination, sub-optimal vaccine responses, and distinct mechanisms of vaccine-induced protection. To address these challenges, experts convened at the 4th International Precision Vaccines Conference in Rome. Co-hosted by the Precision Vaccines Program of Boston Children's Hospital and Ospedale Pediatrico Bambino Gesù, the conference focused on biomarkers of vaccine safety and efficacy in vulnerable populations. Discussions at the conference emphasized the need for multidisciplinary strategies and international collaborations to optimize vaccine development. Key areas of focus included assessing vaccine safety, defining biomarkers for vaccine immunogenicity, developing human in vitro assay models, and accelerating the selection of novel vaccine formulations and adjuvants tailored for vulnerable populations. The conference provided a platform for experts from diverse fields, including immunology, paediatrics, and vaccinology, to exchange ideas and advance research in precision vaccines. This manuscript highlights key concepts discussed at the conference and underscores the importance of precision vaccines in addressing the unique needs of vulnerable populations.