Delta variant of SARS-COV-2 has overtaken all other variants and become a dominant pandemic driver aggressively. In India, it has evolved and yielded delta1, delta2, delta3 and delta4 subvariants. Delta1 has also gradually become the dominant pandemic driver there and across Europe, raising the question whether this is true in other regions around the world. Here I demonstrate that delta1 has also become the dominant pandemic driver in the USA. In April and May 2021, alpha variant was the major pandemic driver, with Ida and gamma variants playing minor roles. Delta variant only started to emerge in April and May, but it rose exponentially and became a major driver one month later. By September, it was detected in ~99% COVID-19 cases and emerged as almost the sole pandemic driver. In the country, ~50% of its population was fully vaccinated in the summer of 2021; vaccination may have selected against all other variants and thereby helped delta variant achieve such an alarming status. One puzzling question is what genomic features make delta variant so highly competitive. Related to this, delta1, but not delta2, delta3 and delta4, has risen exponentially after May 2021, suggesting that unique NSP3 and nucleocapsid mutations that delta1 carries make it so competitive as a predominant pandemic driver. These results indicate that it is not delta variant per se, but its offspring, delta1, that makes delta variant a predominant pandemic driver. Alarmingly, delta1 subvariant has evolved further and gained additional mutations to finetune functions of spike, nucleocapsid and NSP3 proteins. Compared to delta1, delta2 subvariant is less important in driving the ongoing pandemic in the USA, but this subvariant has also evolved further and gained extra mutations. These results suggest a continuously branching model on delta variant evolution and reiterate urgent need to track and block the evolution.