Evidence from multiple sources shows that the Delta variant did, in fact, surge in the U.S. in late summer and fall 2021, and that the worst impacts were concentrated in regions with low vaccination rates.

Key points:

1. Timing and geography of the Delta wave

   • CDC data and contemporaneous reporting show that the Delta variant drove a major U.S. COVID‑19 surge starting in July 2021 and extending into the fall, with substantial case waves in many states.
   • Southern and some Midwestern states—such as Florida, Louisiana, Mississippi, Arkansas, Missouri, and later parts of the Mountain West—experienced particularly severe waves during late summer and into fall 2021.

2. Correlation with low vaccination rates

   • Throughout mid‑2021, those same states had below‑average vaccination coverage compared to the national mean. Public health analyses done at the time repeatedly highlighted that Delta surges were worst in areas with lower vaccination rates (often rural counties and states with lower overall uptake).
   • News and CDC commentary emphasized that Delta was "ripping through" or "sweeping through" under‑vaccinated communities, matching the gist of the prediction (rapid spread in low‑vax regions in that timeframe).

3. Match to the prediction

   • Prediction: “In fall 2021, the Delta variant … will sweep through areas of the country that aren’t vaccinated.”
   • Outcome: By fall 2021, Delta had already driven large case waves in precisely such areas, and continued to circulate heavily there during that period. The direction, timing (late summer/fall 2021), and conditional factor (low vaccination) all align.

Given that Delta did cause a substantial wave concentrated in under‑vaccinated U.S. regions during late summer and fall 2021, this prediction is best classified as right.