When Will the Delta Surge End?

The United States has entered the fourth wave of the pandemic — or fifth, depending on which expert you ask. As the vaccination campaign lags and the contagious Delta variant spreads, cases and hospitalizations are at their highest since last winter. Covid-19 deaths, too, are on a steady incline.

After every other peak has come a trough, however, often for reasons that were not immediately obvious. In Britain, where the variant is also the dominant form of the coronavirus, daily cases fell from a peak of 60,000 in mid-July to half that within two weeks, though they have since been climbing again.

In India, the numbers spiked to more than 400,000 daily cases this spring; experts estimated that the true figure could be more than 20 times greater. The unimaginable toll shocked many who had declared that the country had successfully eluded the virus. But then, in June, infections fell drastically.

Scientists are struggling to understand why Delta outbreaks in those countries dissipated, even if temporarily, and what that may mean for similar surges, including the one in the United States.

In the United States, the variant’s pace has slowed, and new infections are falling in some states, like Missouri, that Delta struck hard. The number of infections over the last week is now 14 percent higher than it was two weeks ago, a fraction of the rate during much of July and early August.

Is the Delta surge beginning to slow in the United States? Or is the variant putting the country on course for months of bumps and valleys?

Expert opinion varies widely on the direction of the virus in the coming months. A number of national forecasts being tracked by the Centers for Disease Control and Prevention predict that cases will rise in the early weeks of September — but many foresee the opposite.

“Whatever downturn we have, I think, will be fairly mild,” said Dr. Celine Gounder, an epidemiologist and infectious disease specialist at Bellevue Hospital Center in New York. “We’re right at that tipping point where back-to-school will tip us back into growth at some point.”

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

“I don’t think that means that everybody who was susceptible has been infected,” she said. “I think people tend to have a false sense of security about that.”

As schoolchildren and some office workers begin mixing in greater numbers this fall, for example, resurgences may appear, she said.

Other epidemiologists said that they were encouraged by trends in Southern states where schools had already opened, noting that while infections were growing among children, they were also falling off among adults.

It is important “not to overly extrapolate” from Delta’s course through Britain and India, Dr. Gounder added. The three countries vary greatly in the percentage of population vaccinated, the ages of the vaccinated, the embrace of large gatherings and open schools, and the prevalence of mask-wearing and other precautions.

Even the weather may be playing a role. Britain’s infections thinned in the summer, while the United States is heading into fall with a huge burden of cases. More people gathering indoors will mean more opportunities for the virus to spread.

Delta’s path across the United States has depended heavily upon vaccination rates, social behaviors, the weather and various levels of precautions, epidemiologists said. Week on week, cases are now falling in a number of Southeastern states and California, but rising across much of the Midwest and Northeast.

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

This makes the variant particularly adept at exploiting opportunities for transmission — the crowded nightclub, the classroom with an unmasked teacher. But it also means that even modest restrictions, like masking and distancing, can bring numbers back down.

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In the Netherlands, where 62 percent of the population is fully inoculated, cases went up by 500 percent after the country removed restrictions. That forced the government to reintroduce some measures, including closing nightclubs and limiting the hours for indoor dining, leading to a rapid decline in new cases.

By mid-June, three weeks into Britain’s Delta surge, cases stopped climbing as quickly as they had been, said Paul Hunter, a professor of medicine at the University of East Anglia and an adviser to the World Health Organization on Covid-19.

One major event changed all that: the European soccer championships, a monthlong series of matches featuring an unusually strong England team that filled pubs and living rooms beginning in mid-June. Cases soared among the young and unvaccinated, especially among men.

“Because Delta is so much more infectious, it tends to move through communities much more quickly,” Dr. Hunter said. “And if it’s more infectious, it’s going to gobble up the remaining susceptible people more quickly.”

Britain is now nearing a point of what Dr. Hunter described as endemic equilibrium, where cases level off as more people develop immunity from vaccination or past infection, even as others see their levels of protection gradually fade.

Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health, said, “Once the schools close, once the football is over, and after the regular contact networks reassert themselves, then Delta has fewer places to go and it slumps.”

But Dr. Hunter cautioned that the point at which infections stabilize remained highly dependent on how and where people mixed, as well as the season.

Understand Vaccine and Mask Mandates in the U.S.

    • Vaccine rules. On Aug. 23, the Food and Drug Administration granted full approval to Pfizer-BioNTech’s coronavirus vaccine for people 16 and up, paving the way for an increase in mandates in both the public and private sectors. Private companies have been increasingly mandating vaccines for employees. Such mandates are legally allowed and have been upheld in court challenges.
    • Mask rules. The Centers for Disease Control and Prevention in July recommended that all Americans, regardless of vaccination status, wear masks in indoor public places within areas experiencing outbreaks, a reversal of the guidance it offered in May. See where the C.D.C. guidance would apply, and where states have instituted their own mask policies. The battle over masks has become contentious in some states, with some local leaders defying state bans.
    • College and universities. More than 400 colleges and universities are requiring students to be vaccinated against Covid-19. Almost all are in states that voted for President Biden.
    • Schools. Both California and New York City have introduced vaccine mandates for education staff. A survey released in August found that many American parents of school-age children are opposed to mandated vaccines for students, but were more supportive of mask mandates for students, teachers and staff members who do not have their shots.  
    • Hospitals and medical centers. Many hospitals and major health systems are requiring employees to get a Covid-19 vaccine, citing rising caseloads fueled by the Delta variant and stubbornly low vaccination rates in their communities, even within their work force.
    • New York City. Proof of vaccination is required of workers and customers for indoor dining, gyms, performances and other indoor situations, although enforcement does not begin until Sept. 13. Teachers and other education workers in the city’s vast school system will need to have at least one vaccine dose by Sept. 27, without the option of weekly testing. City hospital workers must also get a vaccine or be subjected to weekly testing. Similar rules are in place for New York State employees.
    • At the federal level. The Pentagon announced that it would seek to make coronavirus vaccinations mandatory for the country’s 1.3 million active-duty troops “no later” than the middle of September. President Biden announced that all civilian federal employees would have to be vaccinated against the coronavirus or submit to regular testing, social distancing, mask requirements and restrictions on most travel.

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