March 7th, 2021 Daily Covid Cases Plummet below 50,000 for first time since Fall of 2020.

CDC Data Tracker ^ | 3/9/2021 | CDC Website

[1Old Pro]

41,675 on March 7th, 7 day trend continues to plummet. New Daily deaths, 877.

[2aProtectsTheRest]

"Probably a few factors contributed to the big decline since January 8th (like growing natural immunity from exposure)"

It's the same thing that happened after April 8th and July 22nd: the current wave crested. At the micro level, this indicates that the cohorts mixing over the various holidays (Halloween, Thanksgiving, Christmas, and New Year's) had spread it amongst each other to the extent that mixing allowed. With that spread completed, the number of new available hosts declined and the new case numbers declined as well. As you pointed out, the decline in new cases began well before Biden took office. It began while President Trump was in office. The wave began because households mixed significantly more during the various holidays and the wave ended when cohort mixing declined.

I don't know why people keep expecting some exponential explosion of cases that ends in a sudden magical instant called "herd immunity". That's not how any disease has ever worked in the history of the world. The Spanish flu in 1918 had 4 distinct waves. And it did so without Joe Biden or President Trump. Diseases go in waves. And herd immunity doesn't mean what people think it means either. It means Rt < 1.