What does the Delta variant have in store for the United States? We asked coronavirus experts

The United States is standing at a dire inflection point, with pandemic coronavirus cases surging and only 50% of the population fully vaccinated. Driving the latest wave is the highly contagious Delta variant, which according to the Centers for Disease Control and Prevention (CDC) caused between 80% and 87% of all U.S. COVID-19 cases in the last 2 weeks of July—up from 8% to 14% in early June. The variant’s exceptional infectiousness has driven cases from a 7-day average of 13,500 daily cases in early June to 92,000 on 3 August. At the same time, an internal CDC document that leaked last week says the variant may make people sicker, citing published reports from Singapore and Scotland and a preprint from Canada.

The good news is that severe disease and death are highly unlikely among the vaccinated—and U.S. vaccination rates are beginning to increase once again, if modestly.

How bad will the U.S. surge become, and how long will it take to recede? “Anyone saying they know exactly what is happening is overconfident,” says Natalie Dean, a biostatistician at Emory University. “There is a lot of uncertainty about what will happen in the future, even on a relatively short time scale.” With that proviso, here is what Dean and other scientists closely following the pandemic told Science they foresee.

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How many cases and deaths can the United States expect in the Delta-driven surge?

Many computer models predict case counts will peak sometime between mid-August and early September. That peak may bring as many as 450,000 daily cases, according to forecasters at the Institute for Health Metrics and Evaluation (IHME) at the University of Washington. IHME’s models have drawn controversy throughout the pandemic and other groups are more conservative: The COVID-19 Forecast Hub predicts the daily case count on 21 August will be somewhere between 29,000 and 176,000, using a range of estimates from 41 different models.

But all projections rely on assumptions that are moving targets—such as mask wearing and vaccination behavior—and accuracy quickly diminishes the further out the forecast. “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 the Johns Hopkins Bloomberg School of Public Health.

Hospitalizations and deaths will lag behind cases by several weeks and, given the number of people who are now vaccinated, deaths are expected to be lower than their peak of more than 3400 per day in January. Still, IHME researchers forecast a peak of about 1000 deaths a day in mid-September, and a total of 76,000 additional deaths by 1 November. But if 95% of the people in the United States wore masks, their model predicts, 49,000 of those lives would be saved. Americans, including young and healthy people, should not underestimate this variant, warns Ali Mokdad, an IHME epidemiologist. “Delta is a nasty one,” he says.

Can other countries’ Delta surges offer hints about what will happen in the United States?

In India, where the Delta variant was first identified, a massive, Delta-driven wave began in late March and receded by late June, even though mask wearing was spotty and less than 1% of the population was vaccinated as the wave began. In the United Kingdom, a surge that began in early June peaked in mid-July and is now rapidly receding, although daily cases are still many times what they were before the Delta variant took over.

But assuming the U.S. surge will recede as quickly as the one in the United Kingdom did may be a mistake. In that country, vaccine uptake has been much higher than in the United States. Former CDC Director Tom Frieden, president of the nonprofit Resolve to Save Lives, cautioned yesterday in a tweet that the number of unvaccinated Americans could make the U.S. surge “much deadlier” than the United Kingdom’s.

The national case count obscures the record-breaking infection rates in U.S. states with low vaccination rates such as Florida and Louisiana. In Alabama, where just 35% of the population is fully vaccinated, hospitalizations have more than doubled in the past 10 days to nearly 1700—the same number of full beds as at the end of November 2020. The difference: Back then, the doubling took 6 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, Birmingham (UAB). What’s more, he says, the median age of patients at the UAB hospital has fallen from 65 to 52 since January, and healthy young adults are starting to turn up in the intensive care unit.

Can Delta infect fully vaccinated people?

Yes, although vaccination still protects extremely well against severe disease and death. A study of a recent Delta-driven outbreak in Provincetown, Massachusetts, published in CDC’s Morbidity and Mortality Weekly Report last week, was pivotal to the agency’s decision to reverse itself and advise fully vaccinated people to wear masks in indoor public places in areas where transmission is high, agency Director Rochelle Walensky said. In the Massachusetts outbreak, fully vaccinated people accounted for 74% of nearly 469 COVID-19 cases. (Four of the five people hospitalized in the outbreak were fully vaccinated; no one died.)

Strikingly, the study found that fully vaccinated people carried just as much virus in their noses and throats as the unvaccinated. Since then, a new, not-yet-peer-reviewed preprint from the University of Wisconsin, Madison, has reported similar findings.

It’s not surprising that, at the time they’re diagnosed, fully vaccinated people might carry a large nasal load of a variant that’s known to replicate rapidly, says Sixto Leal, who directs medical microbiology at UAB hospitals. That’s because although vaccines are excellent at generating blood-borne antibodies, they are not as good at generating a form of antibody that occupies the lining of the nose and throat. “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.

But in vaccinated people, the replication soon alerts the immune system to send blood-borne antibodies that neutralize the virus in the nose and throat, Leal says. Another new preprint from scientists in Singapore found that although vaccinated and unvaccinated patients infected with Delta had similar viral loads when diagnosed, those loads declined more rapidly in the vaccinated. “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 might happen in the fall and winter, when people move indoors and children are back in 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 influence the debate over whether people should receive booster doses?

Scientists generally agree on the need for immunocompromised people to receive boosters soon, although a go-ahead from U.S. regulators will be needed. Israel is moving ahead with administering a third vaccine dose to people ages 60 and older, and the United Kingdom may soon follow with boosters for older people.

But experts disagree on whether Delta’s emergence calls for an urgent focus on boosters in the general population. Pfizer added fuel to the conversation last week, when it posted a preprint showing the efficacy of its vaccine declined from 96.2% to 83.7% more than 4 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 will make the Delta surge fade away?

Although the reasons for Delta receding in India and the United Kingdom remain unclear, increasing population immunity—from either infection with the virus or vaccination—should give the variant increasingly fewer new opportunities to spread. Human behavior plays a role, too. “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 may bend the U.S. curve is CDC’s revised recommendation last week that fully vaccinated people again wear masks in public, indoor spaces in areas of high transmission. The decline will take time. “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 never stopped wearing a mask in indoor settings with other people, now says he is not frequenting crowded outdoor spaces either—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.”

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