New York held a ticker-tape parade Wednesday for the health care workers and others who helped the city pull through the darkest days of COVID-19, while authorities in Missouri struggled to beat back a surge blamed on the fast-spreading delta variant and deep resistance to getting vaccinated.

The split-screen images could be a glimpse of what public health experts say may lie ahead for the U.S. even as life gets back to something close to normal: outbreaks in corners of the country with low vaccination rates.

"We've got a lot to appreciate, because we're well underway in our recovery," declare...