[Comment] COVID-19 vaccines in the age of the delta variant

The clinical development and global rollout of highly effective vaccines against SARS-CoV-2 has been unprecedentedly rapid. Nevertheless, viral evolution has continued at pace to the extent that questions are being raised about the continued effectiveness of first-generation vaccines in the face of variants of concern (VOCs). In particular, the rise of the delta variant (B.1.617.2) to become the dominant virus in most of the world has spurred efforts to assess vaccine effectiveness against VOCs and to understand the associated immune mechanisms of protection.