Delta Varient Surges in US

In June, U.S. coronavirus cases were at a low point not seen since the beginning of the pandemic, and nearly half the population had received at least one shot. States lifted virtually all virus restrictions and people relaxed their behavior in celebration.

The timing proved disastrous, especially for areas with lower vaccination rates. Another variant, this time Delta, took hold and quickly grew to account for a majority of U.S. cases. Missouri saw the first big surge of the Delta wave.

“That’s where the fire was ignited; then the fire started to spread to other places,” Dr. Vespignani said.

Soon, that outbreak moved across Arkansas, then Louisiana, both states with low vaccination rates. Florida became another early Delta hot spot. By the end of August, most states in the South had hit new records for daily cases or deaths and the virus turned northward, causing surges in the upper Midwest and Mountain West.

While the Delta wave rolled across much of the country, some places were relatively spared.

“That fire was never able to get, for instance, into the Northeast corridor,” Dr. Vespignani said. “It’s where there’s one of the highest vaccination rates. It’s like there’s a wall.”

Some experts say that the vaccination campaign and much of the country having already experienced several waves of outbreaks — which have conferred some immunity to those who were infected and recovered — have made them cautiously optimistic for the winter.