Every few months, the news seems to have a story about some new strain of SARS-CoV-2, the coronavirus that causes COVID-19. From Delta to Omicron and now beyond to Omicron’s numerous subvariants, SARS-CoV-2 has been changing ever since it first came to our attention in late 2019. But where do these viral strains and variants come from? When should they be concerning? How do viral changes influence ongoing vaccination efforts?