As the delta variant spreads through the United States and Covid-19 cases spike, health policy officials and public health experts are split on when they believe the surge will start to decline, with some predicting it could wind down this month and others hypothesizing it won't peak until October.

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When will the delta surge end?

For instance, former FDA commissioner Scott Gottlieb in late July said he believes the United States will follow the trajectory of the United Kingdom, where a delta-driven surge has already started to decline. Based on that model, he estimated that the United States will start seeing Covid-19 cases decline within the next few weeks.

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"If the U.K. is turning the corner, it's a pretty good indication that maybe we're further into this than we think and maybe we're two or three weeks away from starting to see our own plateau here in the United States," he said.

However, Gottlieb cautioned that his prediction was subject to change, and he expressed concern that schools reopening could lead to Covid-19 cases rising again.

"We'll be turning a corner right at the point that schools start to reopen, and my concern is that schools reopening could cause sort of a second bump in cases," he said. "That remains to be seen."

Separately, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and chief medical advisor to the White House, said he believes the United States will follow a similar trajectory to the United Kingdom.

"Since an acceleration of vaccines doesn't give a result until several weeks after, we are already on a trajectory that looks strikingly similar to the sharp incline that the U.K. saw," he said.

However, Fauci added that he predicts the United States will see "between 100,000 and 200,000 cases before this thing starts to turn around."

Meanwhile, Ali Mokdad, chief strategy officer for population health at the University of Washington (UW), said he predicts Covid-19 cases "will peak mid-August" and then decline. He cited estimates from UW's Institute for Health Metrics and Evaluation, which projects in its "most likely" scenario that Covid-19 deaths will peak around 1,000 per day in mid-September.

However, other public health experts predict a later peak of cases, including Eric Topol, founder and director of the Scripps Research Translational Institute, who said he believes cases will start to decline in September.

"I'm hoping we get over this delta hump [by then]," Topol said. "But sometimes, I am too much of an optimist."

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Still other experts—including researchers at the Covid-19 Scenario Modeling Hub—projected the delta surge will continue to increase throughout the summer and fall, ultimately peaking in mid-October.

And while all the experts cautioned that their projections were estimates and not guarantees, Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security, declined even to provide an estimate. Noting that it's difficult to know when the delta surge will peak given how cases are undercounted, Adalja said he predicts there will be a decline in cases only when "more people get infected and develop natural immunity."

The vaccination factor—and a potential 'game changer'

However, Fauci and other experts noted that—among other factors influencing the trajectory of the delta-driven surge, such as social distancing and masking—much of delta's future depends on whether more people get vaccinated. Currently, according to CDC data, 58.2% of the U.S. population has received at least one dose of a Covid-19 vaccine and 49.9% are fully vaccinated.

"What we're seeing, because of this increase in transmissibility, and because we have about 93 million people in this country who are eligible to get vaccinated who don't get vaccinated—that you have a significant pool of vulnerable people," Fauci said.

"Where you're seeing a lot of infection, the rate of vaccination as an average is better than the rest of the country," he added. "That's telling us that the states that are suffering most from the increase are starting to realize that you've got to get vaccinated if you want to get out of this."

But experts are also questioning what proportion of the population needs to be vaccinated to reach herd immunity—and when we can hit that critical share.

Previously, experts suggested herd immunity would be reached with 70% to 80% of the population vaccinated. But Ricardo Franco, from the University of Alabama at Birmingham, said the number may need to be closer to 90%, given the infectiousness of the delta variant.

Justin Lessler, an epidemiologist at the University of North Carolina, agreed. "I am relatively confident that if we could get 90% or more of the eligible population vaccinated that we would see the epidemic begin to resolve," he said.

But when that might happen is still up in the air, Topol said, noting that it's difficult to predict when and whether vaccination rates will increase.  Some of the unvaccinated may feel like soldiers in a foxhole, rushing to get vaccines as soon as possible, he said. But others may hear reports of breakthrough infections among the vaccinated and wonder why they should bother getting a shot.

That said, according to Fauci, one potential "game changer" will be when FDA fully approves the Covid-19 vaccines. As of now, the vaccines are under an emergency use authorization.

Once FDA gives the vaccines full approval, "those people who are hesitant to get vaccinated because they perceive the emergency use authorization as not being proof enough that it's safe and effective—even though we have ample, ample evidence that it's highly effective and highly safe—I think you're going to see more people get vaccinated," Fauci said. (Doheny, WebMD Health News, 8/4; Firth, MedPage Today, 8/4; Lonas, The Hill, 8/4; Stankiewicz, CNBC, 7/26)