Delta on the Decline? Not So Fast

It’s a familiar refrain, those prominent voices claiming we’re all but done with the pandemic of the century.

Former FDA commissioner Scott Gottlieb, MD, told the New York Times this week he thought the U.S. was riding out “the last major wave of infection.” Writer David Leonhardt concluded in the same newsletter, “the worst of the pandemic is almost certainly behind us.”

But public health experts say, “Not so fast.” While cautiously optimistic, they point to other premature forecasts of the pandemic’s end, and say it’s impossible to predict what will happen in the coming months — particularly with the potential for new variants, lackluster vaccination rates, and colder weather on the way.

“I don’t know how people could possibly know that,” Leana Wen, MD, of George Washington University, told MedPage Today. “If there’s anything that we’ve seen from COVID thus far, it’s that it’s extremely difficult to prognosticate.”

New daily infections in the U.S. have plunged from a September 1 peak, and COVID-19 hospital admissions are down by 19% as of October 5, according to HHS data. Worldwide, new cases also appear to be decreasing in the countries most affected by COVID.

But this isn’t the first time a shift in numbers has lulled leaders into a false sense of safety, experts said.

“Last May and June, we celebrated too early, and look what happened,” Wen said. “I really don’t want to see this happen again.”

“We’d all like it to be true,” Stephen Morse, PhD, an epidemiologist at Columbia University Medical Center, said in an email to MedPage Today. “There were similar conjectures before the Delta variant appeared and knocked all our assumptions for a loop.”

This time around, the U.S. does have the advantage of vaccination, experts noted.

“The fact that the surge was muted, [it was] much less than prior ones, you can largely attribute to how many people are vaccinated,” said Joseph Eisenberg, PhD, MPH, an epidemiologist at the University of Michigan. “It’s very, very clear that the vaccine has impacted transmission.”

Eisenberg also said the widely reported “2-month cycle” of case spikes and decreases seems to be at play here, but noted it’s difficult to know exactly why the virus follows such a consistent pattern, seemingly regardless of how humans strategize. Part of the cycling is likely tied to the virus exhausting its options, so to speak, in any given community, he said.

To spread, the virus infects all the “susceptibles” in a cluster or geographical area, and then moves on to the next. That’s why some geographical areas are still exceptions to the overall decline, with cases surging and hospitals still severely strained. Successful spreading to many clusters leads to an exponential increase in cases, but when enough people are immune in those clusters, there’s a large overall decline.

“I think the most likely possibility is that the higher transmissibility of the Delta variant led to many susceptible people becoming infected and developing immunity,” Morse said, calling this the “optimistic” take.

However, in the U.K., after an initial decline in the summer’s Delta wave, case levels started to plateau, and it remains to be seen whether the U.S. will follow a similar pattern.

The most crucial factor besides the natural ebb and flow of the virus is human behavior, and U.S. vaccination rates — perhaps the most important human factor of all — aren’t where public health experts would like them to be.

“Unfortunately, we still have about 70 million people in the U.S. who are eligible to be vaccinated but are not. So I don’t know that we have enough immunity to stabilize another surge,” said Wen.

Then, there’s the very real possibility of another dangerous variant. “We don’t know whether that will happen, but we weren’t expecting Delta, either,” Morse wrote.

Sen Pei, PhD, who studies the transmission dynamics of infectious disease at Columbia’s Mailman School of Public Health, wrote in an email that in addition to the risk that humans will loosen up on precautions, the role the season change will play is still uncertain.

“The answers to some of the questions are inherently unpredictable,” he wrote. “So it may be too early to conclude that there will be no winter waves.”

Viruses survive better in cooler and drier weather, and people also meet indoors much more frequently in the fall and winter, he said. Gathering-heavy holidays further compound the problem of close social contact, increasing everyone’s chances of spreading COVID.

With so much uncertainty, public health experts and epidemiologists stressed that safety measures are as important as ever. Though we’re in a “much better place,” Eisenberg said, “we want to also be diligent and not be too careless.”

That includes continuing to socially distance and mask up in crowded areas, but most importantly, he said, getting vaccinated.

Wen also called for increased COVID testing for students and workers: “Testing has once again been forgotten and that’s a major mistake,” she said. “Other countries have it, why can’t we?”

Ultimately, experts said there’s no way to know what COVID has in store for us.

“I’d like to be optimistic, but it’s too soon to throw away our masks and vaccines,” Morse wrote. “Sadly, much of this could probably have been prevented by some fairly simple public health measures if they were started early and everyone took the epidemic seriously.”