In the first big test of Covid- 19 vaccines during a Covid-19 surge, places with higher vaccination rates are dodging the worst outcomes so far, while cases and hospitalizations surge in less-vaccinated areas.

There are more tests yet to come, including when cold weather forces people in the well-vaccinated Northeast back indoors. But as the highly contagious Delta strain tears through the country, the trends thus far suggest vaccines can turn Covid-19 into a less dangerous, more manageable disease.

“Vaccines definitely make a difference,” said David Dowdy, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health.

A Wall Street Journal analysis shows sharp geographic divides in vaccination and hospitalization levels, with every state that has an above-average vaccine rate showing below-average hospitalizations, including in well-vaccinated New England. In the South, meanwhile, fewer people are vaccinated on average and hospitalization rates are climbing faster.

The Delta-driven surge is unlike its predecessors in the U.S. because the variant spreads more easily and because it is confronting a partially vaccinated population. The U.S. needed an extra month to reach President Biden’s goal of getting 70% of adults at least one shot by July 4. While vaccination rates are picking up, most states remain behind that mark.

The Centers for Disease Control and Prevention says fully vaccinated people are much less likely to be hospitalized or die than people with similar risk factors who aren’t vaccinated. Vaccines also reduce the risk of fully vaccinated people getting infected or transmitting the virus, the agency says, though somewhat less so against Delta than previous variants.

Delta’s relatively recent emergence and other complications, like slowed data collection in many states, have made it difficult to fully understand how the connection between new cases and deaths may be changing, health experts say.

Delta’s earlier surge in the U.K. offers some clues. In January, when cases in the largely unvaccinated U.K. peaked near 60,000 a day, daily mortality counts topped 1,200. More recently, while the U.K. has yet to see the full effects of a case surge that neared 48,000 in late July, deaths are hovering around 80 a day.

Cases have been rising around the U.S. this summer as Delta became the dominant Covid-19 variant, but effects including rising mortality levels have been most pronounced in counties with weaker vaccination rates. Vaccination rates are higher in parts of the country that tend to experience colder winters and saw less transmission last summer, such as New England, which epidemiologists think could be under more pressure this fall.

The people who have been most at risk from Covid-19 are now the most inoculated: According to the CDC, 80% of people 65 and older are fully vaccinated, compared with about 50% of the total population.

This focus on vaccinating the elderly first has appeared to reduce Covid-19 related deaths. In December, during the winter surge and before mass vaccination, Americans ages 75 and up represented more than 63% of Covid-19 deaths for several weeks. More recently, the number has hovered near 40%, CDC data show.

“The elderly are greatly protected by higher vaccination rates, and the population that is getting overwhelmingly sick are the younger folks,” said Manoj Jain, an infectious disease physician at the Rollins School of Public Health at Emory University.

During the Delta surge, however, younger people who aren’t as thoroughly vaccinated are flooding into hospitals. In states with more than 25 hospitalizations per 100,000 residents over the past week, hospitalization numbers are at roughly the same levels they were at peaks in January, but ages are different.

“What the data tell me is even if I were 22 years old, I’d be scared if I wasn’t vaccinated,” said Philip Landrigan, an epidemiologist who directs the Program for Global Public Health and the Common Good at Boston College. The 79-year-old is fully vaccinated.

Even though vaccinated people can catch and in some cases spread Covid-19, data suggest vaccines provide protection against infection and transmission. In Santa Clara County, Calif., the recent seven-day average of daily cases among vaccinated people was roughly 6.8 per 100,000 people on Aug. 4; the case rate among the unvaccinated was nearly four times higher.

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Data from about half of states on Covid-19 infections among vaccinated people, collected by the Kaiser Family Foundation, found that less than 1% of fully vaccinated people have reported experiencing a breakthrough infection in 2021.

“Vaccines are still excellent at protecting against serious illness and death,” said George Han, deputy health officer for Santa Clara County. “They also provide significant protection against infection, though perhaps not quite as effective against the Delta variant.”

Loosened restrictions, less mask-wearing, summer travel and people heading inside in the hot-weather South are factors—along with a slowed vaccination push—that made it easier for Delta to spread, epidemiologists say. Prior infections also confer some level of protection against the virus, but health officials and immunologists still recommend vaccines for recovered people to boost their immune responses.

If there is a silver lining, the Delta-driven outbreak appears to be spurring on more people to get vaccinated, particularly in states hit hard by recent surges.

Delta’s heightened contagiousness is one reason epidemiologists and health authorities say that so-called herd immunity, or societal protection against the virus, is likely to elude the U.S. They say vaccines still make Covid-19 a less dangerous and more manageable disease for the American public.

“Ultimately, what we want to do is prevent people from getting sick, and death, and disability from this virus,” said Wafaa El-Sadr, a professor of epidemiology and medicine at Columbia University. “Prevention from infection is kind of the cherry on the cake.”

Write to Brianna Abbott at brianna.abbott@wsj.com and Jon Kamp at jon.kamp@wsj.com