Delta Variant: How bad will the U.S. surge become, and how long will it take to recede?

There are more pandemic coronavirus cases than ever before, and only half of the population is fully vaccinated. The highly contagious Delta variant caused between 80 and 87 percent of all US COVID-19 cases in the last two weeks of July, up from 8 to 14 percent in early June. Due to the variant’s exceptional infectiousness, cases increased from 13,500 daily in early June to 92,000 on August 3. Meanwhile, an internal CDC document leaked last week says the variant may make people sicker, citing Singaporean and Scottish studies and a Canadian preprint.

The good news is that severe disease and death are extremely unlikely among those who have been vaccinated—and vaccination rates in the United States are beginning to rise again, albeit slowly.

How bad will the surge in the United States become, and how long will it take to subside?

“Anyone saying they know exactly what is happening is overconfident,” says Natalie Dean, an Emory University biostatistician.

“There is a lot of uncertainty about what will happen in the future, even on a relatively short time scale .”

With that caveat in mind, here is what Dean and other pandemic experts anticipate.

How many cases and fatalities should the United States anticipate as a result of the Delta-driven surge?

Numerous computer models forecast a peak in case counts between mid-August and early September. According to forecasters at the University of Washington’s Institute for Health Metrics and Evaluation (IHME), that peak could result in as many as 450,000 daily cases. Throughout the pandemic, IHME’s models have generated controversy, and other groups are more conservative: The COVID-19 Forecast Hub forecasts that the daily case count will be between 29,000 and 176,000 on August 21, based on a range of estimates from 41 different models.

However, all projections are subject to changing assumptions—such as mask use and vaccination behavior—and accuracy rapidly deteriorates the further out the forecast is made.

“We can probably expect to see cases continue to rise for the next 2 or 3 weeks at least. Beyond that, I think it’s challenging to predict,” says David Dowdy, an infectious disease epidemiologist at Johns Hopkins Bloomberg School of Public Health.

Hospitalizations and deaths will lag several weeks behind cases, and given the increased vaccination rate, deaths are expected to be lower than the January peak of over 3400 per day. Nonetheless, IHME researchers forecast a peak of approximately 1000 deaths per day in mid-September and an additional 76,000 deaths by 1 November. However, their model predicts that if 95 percent of people in the United States wore masks, 49,000 of those lives would be saved. Americans, particularly young and healthy individuals, should not underestimate this variant, warns Ali Mokdad, an epidemiologist with the IHME. “Delta is a nasty one,” he asserts.

Can the Delta surges in other countries provide insight into what will happen in the US?

In India, where the Delta variant was first identified, a massive Delta-driven wave began in late March and dissipated by late June, despite the fact that mask use was patchy and less than 1% of the population was vaccinated at the time the wave began. In the United Kingdom, a surge that began in early June peaked in mid-July is rapidly subsiding, though daily cases remain many times what they were prior to the Delta variant taking over.

However, assuming the surge in the United States will subside as quickly as the one in the United Kingdom may be a mistake. Vaccine uptake has been significantly higher in that country than in the United States. Tom Frieden, former director of the Centers for Disease Control and Prevention and president of the nonprofit Resolve to Save Lives, warned in a tweet that the number of unvaccinated Americans could make the US outbreak “much deadlier” than the one in the United Kingdom.

The US Covid surge continues, but hospitalizations are increasing far faster here than they ever did during the UK’s surge. With so many unvaccinated Americans, our surge will be MUCH deadlier. pic.twitter.com/JNncv5GL25
— Dr. Tom Frieden (@DrTomFrieden) August 3, 2021

The national case count obscures record-breaking infection rates in low-vaccination-rate states such as Florida and Louisiana in the United States. In Alabama, where only 35% of the population is fully vaccinated, hospitalizations have more than doubled in the last ten days to nearly 1700—equivalent to the number of full beds in November 2020. The difference: Historically, doubling took six weeks.

“The amount of time you need to be exposed to someone who has the Delta variant is much less than what it was with that ancestral strain,” says epidemiologist Russell Griffin of the University of Alabama at Birmingham (UAB). Additionally, he notes that the median age of UAB hospital patients has decreased from 65 to 52 since January, and healthy young adults are beginning to show up in the intensive care unit.

Can Delta infect people who are fully vaccinated?

Yes, although vaccination continues to offer excellent protection against severe disease and death. A study of a recent Delta-related outbreak in Provincetown, Massachusetts, published this month in the CDC’s Morbidity and Mortality Weekly Report, influenced the agency’s decision to reverse its previous position and advise fully vaccinated people to wear masks in indoor public places in areas with high transmission, agency Director Rochelle Walensky said. Fully vaccinated individuals accounted for 74 percent of the nearly 469 COVID-19 cases in Massachusetts. (Four of the five people hospitalized during the outbreak received all recommended vaccines; no one died.)

Surprisingly, the study discovered that fully vaccinated individuals carried the same amount of virus in their noses and throats as unvaccinated individuals. Since then, a new, unpublished preprint from the University of Wisconsin, Madison, has reported comparable findings.

It’s unsurprising that fully vaccinated individuals may have a high nasal load of a variant known to replicate rapidly at the time of diagnosis, according to Sixto Leal, director of medical microbiology at UAB hospitals. That is because, while vaccines are excellent at generating blood-borne antibodies, they are less effective at generating antibodies that adhere to the nose and throat lining.

“There’s a window of time when fast-replicating virus can enter [the cells lining the nose], replicate like crazy in a very high amount, and [cause] symptoms,” Leal says.

However, in vaccinated individuals, the replication of the virus quickly alerts the immune system to send blood-borne antibodies that neutralise the virus in the nose and throat, Leal explains. Another new preprint from Singapore scientists found that while vaccinated and unvaccinated patients infected with Delta had comparable viral loads at diagnosis, the vaccinated patients’ viral loads declined more rapidly.

“Based on basic immunology, that’s exactly what we would expect—that vaccinated individuals would clear the infection much faster,” says Kristian Andersen, an infectious disease researcher at Scripps Research.

What happens in the fall and winter, when people return to their homes and children to their school?

“There’s every reason to suspect that SARS-CoV-2 infection rates will be worse in winter as opposed to summertime, because that’s the path we see with other respiratory viruses,” Dowdy says.

“[But] we don’t have evidence yet.” And with so much Delta circulating in both vaccinated and unvaccinated people, we are unwittingly exerting selection pressure for new, more dangerous variants to evolve, Andersen cautions. “This fall and winter I am not sure we will be dealing with Delta. I think we will probably be dealing with a variant we haven’t heard about yet,” he says.

“From a viral evolution perspective, it would be foolish not to expect that.”

Does Delta’s trajectory have an effect on the debate over booster doses?

Scientists are generally in agreement about the urgent need for immunocompromised individuals to receive boosters. Israel has already started with the administration of a third dose of the vaccine to people aged 60 and older, and the United Kingdom may soon follow suit with older adults receiving boosters.

However, experts disagree on whether the emergence of Delta necessitates an immediate focus on boosters in the general population. Pfizer fueled the debate last week when it published a preprint demonstrating that the vaccine’s efficacy decreased from 96.2 percent to 83.7 percent more than four months after full vaccination.

But because the available U.S. vaccines are still highly effective against Delta and the vast majority of serious illness and death is occurring in people who are unvaccinated, “I would strongly prioritize getting more people fully vaccinated than getting booster shots in people,” Dowdy says.

Dean adds that a global view is important: “We live in a world where so many people remain unvaccinated. How do you justify that boost to individuals that [already] have a certain amount of protection?”

World Health Organization Director-General Tedros Adhanom Ghebreyesus threw the organization’s moral authority behind that viewpoint today, calling for a moratorium on booster vaccinations through at least September.

Confronted with the Delta variant, he said, “We cannot accept countries that have already used most of the global supply of vaccines using even more of it, while the world’s most vulnerable people remain unprotected.”

But Andersen, who is calling for rapid development and distribution of Delta-specific boosters, calls the booster-versus-vaccine question a false choice. “We need to do both,” he says. “And that requires warlike efforts which we are not doing right now.”

What can make the Delta surge go away?

Although the reasons for the decline of Delta in India and the United Kingdom remain unknown, increasing population immunity—either through virus infection or vaccination—should provide the variant with fewer new opportunities to spread. Human behaviour also plays a role.

“As cases start to climb … [people] start to think twice about that big party they were going to go to,” Dowdy says.

Another factor that could sway the US curve is the CDC’s revised recommendation last week that fully vaccinated individuals once again wear masks in public, indoor spaces in high-transmission areas. The decline will be gradual.

“People need to recognize that things are going to get worse before they get better,” Dowdy cautions. “But it’s not time to panic in thinking that this is going to be December [2020] and January all over again.”

Have COVID-19 scientists changed their own behavior since Delta emerged?

“I started wearing a mask again at the grocery store,” says Dean, who lives in Gainesville, Florida. “I feel confident in the vaccine. I just am not going out to a bar. But I wasn’t doing that a ton [anyway].”

In San Diego, Andersen, who has never stopped wearing a mask in public places, now says he avoids crowded outdoor spaces, including restaurants.

“We get take-out instead.”

At UAB, Leal required masks for his lab’s employees 3 weeks ahead of a new campuswide mandate.

“We had experienced 2 months of happiness and [a] return to normal,” he says.

“Now, we are much more cautious again.”

Image Credit: GEtty

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