In July 2021, the Delta variant (B.1.617.2) caused a significant increase in new cases and reversed what had been a steady decline since January 2021. CDC saw a rapid and alarming rise in the COVID-19 case and hospitalization rates around the country. Data began to emerge that the Delta variant was more infectious and was leading to increased transmissibility when compared with other variants, even in some vaccinated individuals.

The Delta variant is no longer the predominant variant in the United States. Viruses constantly change through mutation, and new variants of a virus are expected to occur over time. COVID-19 vaccines remain the best public health measure to protect people from COVID-19 and reduce the likelihood of new variants emerging. This includes primary series, booster shots and additional doses for those who need them.