Till April, most Covid-19 cases in the US were of the Alpha variant, apart from the few attributed to Iota (No 2), Gamma, Beta and Epsilon. Then came Delta, but in early May it accounted for barely 1 percent cases. By July-end, it has 94.3 percent share in coronavirus infections in the US. The speed is baffling, making scientists nervous. One even speaks of the ‘Jurassic Park’ moment – “when dinosaurs have all got loose again.” What can explain the rise of Delta over other variants?

It's different
Scientists studying what makes this variant unique are yet to arrive at any conclusion. But in the process what they have found is this: The original strain from Wuhan had a ‘reproduction number’ or R-naught of about 2.5, that is, each infected person was likely to pass it to 2.5 people (not counting immunization and protection protocol). For Delta, R-naught is five, the highest of any variant so far. [https://www.washingtonpost.com/health/delta-variant-most-prevalent/2021/08/08/d1017f0e-f558-11eb-9068-bf463c8c74de_story.html] Beyond that, however, little is said with certainty about it. Does its infection lead to a more server disease? There is not enough data and any answer is anecdotal, though it’s believed to be more severe.

Behavior
Such a contagious variant broke out in the US just when the nation was least prepared. It was just when people were relaxing the Covid-appropriate behavior, crowding public places. Even the CDC was caught unawares, as it advised the mask was no longer necessary for the vaccinated. Delta could then spread much faster than it would have otherwise.

Vaccine resistance
As the pandemic appeared waning, vaccine hesitancy and resistance went up. Of late, there has been resurgence in vaccination, as Delta fears seem to have pushed the fence-sitters. Still, more than 90 million eligible US citizens are going around without a single vaccine shot. [https://www.nationalacademies.org/news/2004/04/90-million-americans-are-burdened-with-inadequate-health-literacy-iom-report-calls-for-national-effort-to-improve-health-literacy] Then there are children who are waiting because the current vaccines do not have approval for that age group. The younger populations seem more vulnerable to this variant than others.

Breakthrough cases
Delta, more than other variants, affected the vaccinated too, though symptoms are said to be milder.

Also see:

https://www.azmirror.com/blog/what-makes-the-covid-19-delta-variant-different-and-dangerous/

https://www.yalemedicine.org/news/5-things-to-know-delta-variant-covid

CDC document on Delta:
https://www.washingtonpost.com/health/2021/07/29/cdc-mask-guidance/