The Nobel Assembly at Karolinska Institutet has awarded the 2023 Nobel Prize in Physiology or Medicine jointly to Katalin Karikó and Drew Weissman. This follows their discoveries concerning nucleoside base modifications that enabled the development of effective mRNA vaccines against COVID-19. Through their ground-breaking findings, they have fundamentally changed the understanding of how mRNA interacts with our immune system, and thus contributed to the unprecedented rate of vaccine development during one of the greatest threats to human health in modern times.