London — A study conducted in the U.K. offers some of the first large-scale, real-world data on how well vaccination protects people against catching a "breakthrough" COVID-19 infection, and how well it protects breakthrough patients from becoming seriously ill. The results are encouraging. The peer-reviewed study published Wednesday in The Lancet medical journal will help policy makers and epidemiologists fill in a significant gap in the understanding of the true efficacy of three of the major vaccines being used worldwide.... The data, gathered from December 8, 2020, through July 4, 2021, show that of more than 1.2 million adults who...