Delta waves in India and the U.K. have already receded. Could the same happen in the U.S.?

August 3, 2021, 6:19 PM UTC

On Friday, the U.S. government reported over 103,000 new COVID-19 infections, the first time in six months that the country has topped 100,000 infections in a single day and over 10 times the number of daily infections the U.S. was reporting just weeks ago.

The recent wave of infections is largely driven by the highly contagious Delta variant of COVID-19 and concentrated mostly among unvaccinated individuals.

The steep rise in infections is prompting local authorities in hard-hit areas to institute new mask mandates and the U.S. Centers for Disease Control to recommend that vaccinated and unvaccinated Americans alike wear masks in indoor areas in some parts of the country—just a month after the U.S. lifted most pandemic restriction measures earlier this summer.

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But data from other countries is providing some hope that the U.S.’ recent Delta-driven wave may dissipate as fast as it emerged. In the U.K., daily confirmed cases of COVID-19 have fallen by 50% in the past week after the country experienced its largest wave of infections in months. In India, where the Delta variant first emerged, confirmed COVID-19 infections have dropped by a factor of ten in the last few months—after peaking at nearly 400,000 daily infections in May. No one knows for certain exactly why Delta cases have dropped so fast—and many experts are baffled.

Andy Slavitt, a former advisor on U.S. President Joe Biden’s COVID-19 response team, recently suggested on Twitter that steep drop-offs in COVID-19 cases in places like the U.K. and India after Delta-driven waves may become a new paradigm for how COVID-19 spreads through communities. The data from the U.K. and India may mean that “Delta takes a quick rise [and] a quick drop,” Slavitt wrote on Twitter.  

Still, experts warn that the U.S.’s large population of unvaccinated individuals means that the recent surge of infections may be far from over. Whether the U.S. will experience a drop-off similar to the U.K. or India remains uncertain as the world attempts to adapt to the new Delta era of the pandemic.

Lessons from India’s second wave

The first “Delta wave” became apparent during the second surge of COVID-19 infections earlier this year in India, where it first emerged. Daily cases skyrocketed from roughly 10,000 in early February to a peak of 414,188 by May 7. But within just a month, cases had dropped nearly 80% to 85,801. On Tuesday, the country recorded 30,549 new cases.

The Delta wave hit India as its vaccination campaign was still in its infancy; only 4% of Indians had received a first dose. As a result, the variant, which made up most of the cases in the second wave, infected millions and killed hundreds of thousands. Oxygen was in short supply, hospitals were overrun, and crematoriums operated at full capacity.

We now know one of the reasons the virus spread so fast: Jeffrey Zients, the White House pandemic response coordinator, said Monday that the Delta variant is more than twice as contagious as the original strain of COVID-19.

While India’s Delta surge seemed to peter out with surprising speed, Jacob John, a pediatrician and virologist who is retired from the Christian Medical College in Vellore, India, says its trajectory in the country “looks like a natural increase and a natural decline.”

“The virus had a free run in India. Nothing was done that made a dent on the transmission,” he said, citing a survey of blood tests that estimates 70% of Indians are positive for COVID-19 antibodies, suggesting widespread contagion. “We are fortunate that the nightmare has come and gone.”

Amit Dutt, principal investigator and scientist at India’s Tata Memorial-ACTREC, a research institute in Mumbai, also cited the blood test study as proof that India had likely reached the 70% threshold for herd immunity, which may have contributed to the second wave’s decline. But he doesn’t consider that finding reassuring.

“India taught the world that herd immunity through natural infection is not an option one can afford,” he said. “The price could be humongous. The learning from India—as a case study—was loud and clear to either vaccinate or face a similar dire consequence.” 

K. Srinath Reddy, president of Public Health Foundation of India, meanwhile, balks at the suggestion that India is already free from the worst of the pandemic.

“We can not say the pandemic has ended. But things have abated in many parts of the country, though not everywhere,” he said.

The southern state of Kerala, for instance, has reported roughly 20,000 cases a day for the last five days, accounting for over half of India’s total cases. Interestingly, the blood test study found that Kerala had the lowest rate of immunity—44%—making it especially susceptible to what experts say may be the early stages of India’s third wave.

Delta’s heightened transmissibility has also forced scientists to reconsider their previous forecasts for reaching herd immunity. In the past, there was consensus among public health officials that 70% protection through natural infection or vaccination was enough to stop the virus from spreading in a community.

But scientists are now questioning that analysis as they learn more about Delta’s fortitude, especially amid new findings that even vaccinated people can spread the variant. Even if 70% of Indians have immunity now, it may not be enough to stave off future waves, especially if the virus mutates further.

“About a third of people in India are still susceptible,” Reddy says. “We have to increase the pace of vaccination. It will be incorrect to assume that we have achieved herd immunity.”

A U-turn in the U.K.

A better comparison for the U.S.’s Delta wave may be the recent surge in the U.K., where COVID-19 vaccines are widely available.

Two weeks ago, England took a gamble: In the face of sky-rocketing Delta variant COVID-19 cases, the British government eased all restrictions and completely opened up its economy (Scotland, Wales and Northern Ireland kept some restrictions).

Despite warnings from scientists, the bid seems to have paid off, as the number of new reported cases peaked on July 17 at 54,674 new reported cases, and then fell steeply. By August 2, new daily cases were less than half that number, with 21,952 new cases reported, according to Public Health England.

Dr. Peter Drobac, a phsycian director at the Skoll Centre for Social Entrepreneurship at the University of Oxford, points out that “the short answer is that nobody knows for sure” what’s behind the drop. But “it is probably a perfect storm of good circumstances.”

Most notably, the U.K. has one of the highest vaccination rates in the world, ahead of the U.S. While the U.S. vaccination rates lie at around 48% fully vaccinated, the U.K. is ahead at 57% of the population. In the U.K., anyone over 18 is entitled to a vaccine, while in the U.S. anyone over 12 can be vaccinated.

What’s more, the U.K. has low vaccine hesitancy, suggesting its vaccination rate will continue to climb. According to the U.K.’s Office for National Statistics, only 6% of the British population is hesitant to get a vaccine, whereas 20% of Americans—when measured across the same time frame—have not been vaccinated and are unlikely to get vaccinated in the future, according to Gallup.

The U.K. also separates itself from its peers by using a track and trace program, which has garnered new attention in recent weeks for the surging number of people being “pinged”—being told by the government they need to self-isolate because they have been exposed to someone with COVID-19. In the last week, the so-called ‘Pingdemic’ has told more than 700,000 people to self-isolate.

Also, “because we are approaching the population immunity threshold, our mitigating behaviors become much more effective,” says Karl J. Friston a neuroscientist at the University College of London. In other words, as more of the population is vaccinated, mask wearing and self-isolating has a far greater impact on suppressing viral transmission, he said.

There are other potential factors, too: the U.K. has managed to sidestep the impact of the potential super-spreader event that was the Euro soccer tournament last month, children are off school on summer holidays, and a heat wave has kept people outside in the good weather, notes Drobac, from the University of Oxford.

While the U.K.’s declining case count is a reason for hope, there are still skeptics. Many, surprised by the fall in cases in the U.K., have suggested it reflects a reduction in testing. William Irving, a professor of virology from the University of Nottingham, suspects that testing rates account for some of the drop off, which he calls the “Trump solution”— if you don’t test anybody, “then you don’t have any cases!”

However, another daily testing numbers show that while testing has declined 12.4% over the week of July 21 to July 27, this doesn’t match up to the 33% decline in COVID-19 cases in the same time period.

The decline in cases has left David Mackie, chief European economist for J.P. Morgan Securities, scratching his head. Mackie said in a market note that while it is hard to fully explain the dramatic collapse in new infections, he is “reluctant to abandon the idea that the Delta variant will be a problem over time,” as its reproduction number is too high and the level of vaccine efficacy is too modest.

But he does note that “it is hard to argue against the idea that the current Delta wave in the U.K. is turning out to be much, much milder than we anticipated.”

Hoping for a ‘hairpin’

Experts in the U.S. don’t necessarily expect a domestic Delta wave to follow the U.K.’s lead, namely because of the difference in vaccination rates. The U.S.’s lagging vaccine campaign may mean the Delta variant has more staying power in the U.S.

“[The Delta variant] has more room to spread, certainly, in unvaccinated populations,” says Ashley St. John, an immunologist at the Duke-NUS Medical School. There are 93 million Americans eligible to receive COVID-19 shots that have opted not to get them.  

St. John said it’s possible that infections are dropping off sharply in the U.S. in places with high vaccination rates because vaccinated individuals who get infected may not carry the disease as long as unvaccinated people.

“A vaccinated individual will have a much shorter duration of infection. So that can lead to these positive cases that come up, but not resulting in as much transmission,” she says.

Christina Ramirez, a biostatistics professor at the University of California Los Angeles, says that the Delta variant has changed how she and other experts model the spread of the disease.

“Vaccines really do help prevent hospitalizations and deaths,” she said. Vaccinated people are ten times less likely to be hospitalized or die than those who are not vaccinated. But she explained that vaccinated individuals are more likely to catch and spread the Delta variant than previous variants of COVID-19.

“Our models need to be revised for this new data reality. We need really good information on the probability of breakthrough infections.”

The uncertainty about how COVID-19 is now spreading makes it difficult for Ramirez and other modelers to tell whether the U.S. is in the early stages of a larger wave or if it will mimic the U.K.’s sharp decline in infections in coming days and weeks.

“I’m really hoping that we get sort of that hairpin [shaped] drop off in infections,” says Ramirez. “But this is really hard to model. For [statisticians] such as myself, this really makes us want to pull our hair out, because it’s really complicated.”

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