Evidence from fall 2021 shows that the SARS‑CoV‑2 Delta variant did in fact sweep through under‑vaccinated parts of the United States and became the dominant strain there.

Key points:

• By early July 2021, Delta was already surging in regions with low vaccination coverage, especially in parts of the South and Midwest, and health officials explicitly warned that areas with low vaccination rates would be hit hardest.
• Through late summer and into fall 2021, the U.S. experienced a major “Delta wave,” with the highest case, hospitalization, and death rates concentrated in states and counties with lower vaccination rates (e.g., many parts of the Southeast and Mountain West). Analyses during that period consistently highlighted a strong geographic correlation between low vaccination uptake and intense Delta transmission.
• By September–October 2021, Delta was the predominant variant in the U.S., and data and reporting showed it driving significant outbreaks specifically in under‑vaccinated communities, matching the prediction that it would “sweep through areas of the country in the fall that aren’t vaccinated.”

Because the prediction is specific (Delta will sweep through under‑vaccinated areas in fall 2021) and that is exactly what occurred, the prediction is right.