The Delta wave of the Covid-19 pandemic is past its peak, with new cases, hospitalizations and deaths declining in most states. The approaching holidays and winter months will test whether the U.S. can sustain that momentum.

New Covid-19 case numbers in the U.S. are close to levels recorded near this time last year, with a seven-day average at about 72,000 a day, Johns Hopkins University data show. But the trajectory is opposite. Last fall, cases were rising while hospitalizations and deaths, trailing indicators, were starting...

The Delta wave of the Covid-19 pandemic is past its peak, with new cases, hospitalizations and deaths declining in most states. The approaching holidays and winter months will test whether the U.S. can sustain that momentum.

New Covid-19 case numbers in the U.S. are close to levels recorded near this time last year, with a seven-day average at about 72,000 a day, Johns Hopkins University data show. But the trajectory is opposite. Last fall, cases were rising while hospitalizations and deaths, trailing indicators, were starting to follow.

Now all those metrics are improving significantly at the national level. Case counts from the Delta surge were declining by mid-September after peaking above 160,000 a day, Johns Hopkins data show. The seven-day new case average was down about 16% this past Tuesday from the week prior, Centers for Disease Control and Prevention Director Rochelle Walensky said, citing CDC data.

Covid-19 hospitalizations, which surged to records in many states as the highly infectious Delta variant spread chiefly among the unvaccinated, are down about 54% from a late-August peak, Dr. Walensky said. The Delta-fueled wave continues to take a serious toll, but the seven-day average in reported deaths has dropped to roughly 1,400 a day from daily averages above 2,000 in late September, Johns Hopkins data show.

“We may be at a turning point,” Marcus Plescia, chief medical officer at the Association of State and Territorial Health Officials, said of the pandemic. “We have a lot more tools now to fight it.”

Public-health experts say factors driving the decline likely include an incremental uptake in vaccines, the return of precautions like mask-wearing in certain areas and growing immunity in the population due to Delta’s rapid spread in hard-hit states including Mississippi and Florida.

“It’s a lot tougher for Delta to hop from person to person because so many people were infected,” said Jason Salemi, an associate professor of epidemiology at the University of South Florida. “It just came at a considerable cost.”

The U.S. hopes to bolster its Covid-19 defenses as more children, such as this girl in Stamford, Conn., get vaccinated.

Photo: Landon Speers for The Wall Street Journal

There are reasons for caution, health authorities say. The U.S. posted its worst numbers of the pandemic, including a quarter-million new cases a day, shortly after the winter holidays last year, when colder weather and family gatherings brought more people together indoors where the virus most easily spreads.

In the U.K., where the vaccination rate is higher than in the U.S., cases remain high after the country bet on immunity from vaccines and prior infections while dropping other measures like indoor crowd limits and mandatory mask-wearing.

Roughly one-third of the U.S. population has yet to get a single vaccine dose. In a few states, including Idaho and West Virginia, roughly half the population hasn’t gotten a shot. In Idaho, the Delta-driven surge appears to be cooling, but hospitals are still in crisis mode and local health officials said they have also seen at least one confirmed case of influenza.

“Whether we’re going to see another spike this fall and winter, I think we’re worried that we can,” Christine Hahn, state epidemiologist at the Idaho Department of Health and Welfare, said at a press briefing this past week.

In Idaho, the Delta-driven surge appears to be cooling, but hospitals are still in crisis mode.

Photo: Kyle Green/Associated Press

New England states Vermont and Maine continue to battle surges as Delta finds pockets of unprotected people there, officials say, despite some of the nation’s highest vaccination rates. In California, what had been a decline in Covid-19 hospitalizations has recently flattened well above a trough hit in June.

Health officials continue to encourage people to get vaccinated against both Covid-19 and the flu, and to take some additional precautions including masking indoors and staying home when they feel ill. More than three-quarters of counties nationwide have what the CDC considers high levels of Covid-19 transmission.

The U.S. could further bolster its defenses as younger children begin to get vaccinated in the weeks ahead. The Food and Drug Administration on Friday authorized the Pfizer Inc. and BioNTech SE vaccine for some 28 million children ages 5 through 11. Shots could be available to children this coming week after a review by the CDC.

Getting shots to eligible adults who haven’t been vaccinated is still a challenge, however, and some health authorities are concerned the Delta wave’s rapid retreat has eliminated some of the urgency that was spurring people to roll up their sleeves.

A New Orleans drive-through Covid-19 vaccination and testing facility in August.

Photo: Bryan Tarnowski/Bloomberg News

In Louisiana, when Delta caused the seven-day new case average to jump to record highs near 6,000 a day in mid-August, state data also show vaccinations increased significantly. But with case averages back down near presurge levels seen in the early summer, vaccination rates have also tumbled. Louisiana is one of the nation’s least-vaccinated states, with just under 48% of the whole population there fully vaccinated, federal data show.

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Louisiana and other states lagging on shots ”will remain vulnerable to a similar surge in the future until we get vaccination rates up,” said Joseph Kanter, the state’s health officer and medical director.

Missouri has also seen vaccination rates wane as its Delta surge faded, state epidemiologist George Turabelidze said. He expects cases to rise again this winter, but said that vaccines and immunity from previous infections should mute the effect.

“Even if future waves happen, they very likely are not going to be of the magnitude that we saw last November and December,” he said.

—Anthony DeBarros contributed to this article.

Recent studies have shown that the effectiveness of Covid-19 vaccines is decreasing, though experts say the shots still work well. WSJ explains what the numbers mean and why they don’t tell the full story. Photo illustration: Jacob Reynolds/WSJ The Wall Street Journal Interactive Edition

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