So far, emerging outbreaks of Covid-19 in Tampa, Fla., Phoenix, Houston and elsewhere in the South have been less deadly than the initial U.S. wave that emerged in March and April. One reason experts cite: Victims in states that raced to reopen bars, restaurants and churches after the initial lockdown tend to be younger and better able to fight off the virus.

Rises in nursing-home cases threaten to break that pattern, a Wall Street Journal analysis of federal nursing-home data shows. Facilities in the Houston and Tampa metropolitan areas marked a nearly 800% cumulative increase in new cases among residents from the last week of May through the week ended June 28, the most recent period available, with more than 400 new cases during that period in both cities. In Phoenix, the increase topped 900%, more than three times the national increase, as nursing homes reported 545 new cases. Data from Miami and San Antonio nursing homes showed fast growth in cases as well.

“We first see it in the community, and then we see it in the residents and staff, and then you see the deaths,” said David Grabowski, a professor at Harvard Medical School who studies nursing homes. “As it spreads to these new states, we’re not that much farther along than we were in the Northeastern states in March and April” in efforts to shield residents from the virus, he said.