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The United States has entered the fourth wave of the pandemic – or the fifth, depending on which expert you ask. Cases and hospitalizations are at their highest since last winter as the vaccination campaign is delayed and the contagious Delta variant spreads. Covid-19 deaths are also on a steady slope.

However, after each peak, a trough often came for reasons that were not immediately obvious. In the UK, where the variant is also the dominant form of the coronavirus, daily cases fell to half that in two weeks from its peak of 60,000 in mid-July, but have been climbing again since then.

In India, numbers rose to more than 400,000 daily cases this spring; Experts estimate that the real figure could be 20 times higher. The unimaginable death shocked many who declared that the country had successfully recovered from the virus. But then, in June, infections dropped drastically.

Scientists are struggling to understand why Delta outbreaks in these countries are dispersing, albeit temporarily, and what this might mean for similar fluctuations, including in the United States.

In the United States, the pace of the variant has slowed, and new infections are falling in some states hit hard by the Delta, such as Missouri. The number of infections last week is now 14 percent higher than two weeks ago, a small fraction of the rate for most of July and early August.

Delta surge starting to slow down in the United States? Or is it the variant that puts the country on course for months of mounds and valleys?

Expert opinion varies widely on the direction of the virus in the coming months. A set of national estimates monitored by the Centers for Disease Control and Prevention It will rise in the first weeks of September – but many predict the opposite.

D., an epidemiologist and infectious disease specialist at Bellevue Hospital Center in New York. “Whatever the crisis we’re going through, I think it’s going to be pretty mild,” said Celine Gounder. “We are at that tipping point where a return to school will lead us to grow again at some point.”

Dr. Gounder predicted that cases in the United States would escalate again in September before decreasing in October. Dr. Gounder said the virus may have burned unvaccinated segments of the population this summer, but other people remained vulnerable.

“I don’t think that means that everyone who is susceptible is infected,” he said. “I think people tend to have a false sense of security about it.”

He said resurgences could occur, for example, as schoolchildren and some office workers began to mingle in greater numbers this fall.

Other epidemiologists said they were emboldened by trends in Southern states where schools were opened, noting that while infections were increasing among children, they were also falling among adults.

Dr. Gounder added that it’s important not to “over-guess” the Delta’s UK and India route. The three countries differ widely in the percentage of the population vaccinated, the age of those vaccinated, the embrace of large gatherings and open schools, and the prevalence of mask-wearing and other measures.

Even the weather may be playing a role. Infections in England declined during the summer months, while the United States fell with a large caseload. More people gathering indoors will mean more opportunities for the virus to spread.

Delta’s path through the United States depends on vaccination rates, social behavior, weather, and a variety of precautions, epidemiologists said. Week after week, cases are currently falling in a number of Southeast states and California, but rising in most of the Midwest and Northeast.

The variant is thought to be more contagious than previous versions of the coronavirus because those infected carry it in significantly greater amounts in their airways.

This makes the variant particularly adept at taking advantage of transmission opportunities such as crowded nightclubs, classrooms with unmasked teachers. But it also means that even modest restrictions like masking and distancing can bring numbers back.

In the Netherlands, where 62 percent of the population is fully vaccinated, cases rose 500 percent after the country lifted restrictions. This forced the government to re-implement some measures, including closing nightclubs and limiting closed meal times, leading to a rapid decline in new cases.

Paul Hunter, professor of medicine at the University of East Anglia and Covid-19 adviser to the World Health Organisation, said in mid-June, three weeks after the UK’s Delta surge, cases stopped climbing as quickly as before.

One big event changed all that: the European football championships, a monthly series of matches featuring an unusually strong England team that filled the bars and living rooms from mid-June. Cases have increased among youth and the unvaccinated, especially among men.

Dr. “Because the delta is so much more contagious, it tends to move much more quickly between communities,” Hunter said. “And if it’s more contagious, it will swallow any remaining sensitive people more quickly.”

England is currently Dr. What Hunter describes as an endemic balance is approaching a point where cases flatten out as more people develop immunity from vaccination or past infections, and others are even seeing protection levels gradually decline.

“When schools close, football ends, and regular networks reassert themselves, Delta has less room to go and collapses,” said Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health.

However, Dr. Hunter cautioned that the point at which infections stabilize is highly dependent on how and where people mix, as well as the season.

New daily cases in the UK have started to climb again after halving from mid to late July. Dr. Hunter said that in recent weeks, the increase was due to outbreaks in regions that had relatively few cases before the pandemic and are therefore naturally less immune, including southwest England and rural areas of Scotland and Northern Ireland.

The variant took a different path in largely unvaccinated India. In the months before the second wave, life had almost returned to normal in many parts of India as cases dwindled and hospitals emptied.

Earlier in March, the government declared that the country was in “the endgame of the epidemic,” and Prime Minister Narendra Modi approved crowded election rallies in several states alongside the Kumbh Mela festival, which attracted millions of devotees. Weddings, cricket matches and family gatherings were in full swing.

In the weeks that followed, millions of people fell ill and thousands died. Hospitals in many major cities suffocated as oxygen and other critical supplies ran out. But cases fell almost as fast as they started, especially in the worst-affected states.

Bhramar Mukherjee, a biostatistician at the University of Michigan, said the decline cannot be attributed to the drop in testing, although the official case count in India is probably underestimated.

Dr. “We predicted that the second wave would always be long and weak,” Mukherjee said. “The higher you rise, the steeper the fall is when you quickly burn off the vulnerable population.”

The antibody test showed that the proportion of Indians infected increased from 21.5 percent in January to 67 percent in July. Antibody tests can be unreliable, but if these numbers are close to right, natural immunity could help the country fend off another dreadful wave.

India may need to rely on natural immunity, as only 9 percent of its population is fully vaccinated, slightly over 50 percent in the United States. For now, cases in India have stabilized at significantly higher levels than previously seen in the pandemic.

The UK has released vaccines by age, with 90 percent of adults getting at least one shot. The strategy has benefited by limiting hospitalizations even as cases increase. Dr. Hanage said the distribution of vaccines in the United States is more erratic, and so will Delta’s rise and fall.

“The nature of delta transmission means that cases will increase in many places at once, but where there are fewer vaccines, the results in absolute numbers will be much, much worse,” he said.

Dr. Hanage warned that reopening schools and offices in the United States would increase cases. Around 10 million adolescents in the country have been vaccinated, which could reduce the impact of school reopening in particular, and more children could be vaccinated this fall.

In previous waves, schools were not hotbeds of infection, but school districts now have fewer restrictions.

Dr. “Many schools across the country are not taking this very seriously this year,” Gounder said. “So you will see the transfer from schools to society.”

Americans should expect to see surges on Thanksgiving, Christmas, and New Year’s as they did last year, he added—although none as badly as last winter.

Dr. “I don’t think we’ll really be around the corner until next spring,” Gounder said.

Apoorva Mandavilli and Benjamin Mueller reported from New York and Shalini Venugopal Bhagat from Goa, India.