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One of the key reasons the Delta variant has ignited new surges of Covid-19 infections across the United States is its remarkable ability to make copies of itself.

That skill has helped make Delta far more transmissible than any other iteration of the coronavirus seen thus far. But its replication prowess could also be at the heart of the other twists Delta has thrown into the pandemic, including the increase in breakthrough infections with the variant and why it potentially causes severe Covid-19 more often.

Delta’s breakneck proliferation isn’t its only trick tied to the increase in infections — and symptomatic infections — in people who’ve been vaccinated. With the mutations the variant picked up as it evolved, it can partially cloak itself from the immune system’s antibodies. That gives the variant a better chance of sneaking past that first line of defense to start an infection compared to earlier forms of the virus. (The vaccines, however, haven’t lost any significant step in protecting against severe outcomes from the coronavirus. They are also still preventing many infections entirely.)

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But experts say a crucial factor in Delta’s spread even to vaccinated people is that those infected with the variant seem to be “shedding” comparatively massive amounts of virus, exposing others to much higher levels. An oft-cited statistic is that, with Delta, people have 1,000 times as much virus in their upper airways early on in their infections compared to with the strain that emerged in China in 2019. Some experts quibble with that exact estimate, but the point remains that people who contract Delta have lots more virus in their noses and throats, and are in turn emitting lots more virus. That influx of virus into others’ airways seems to be able to overwhelm the antibodies that aim to guard the cells from infection.

Jeremy Kamil, a virologist at Louisiana State University Health Shreveport, offered an analogy. “It’s like trying to light a wet pile of hay on fire,” he explained. The body’s immune system is keeping the tinder damp, but if you throw more and more sparks on that wet hay, “eventually one of them catches.”

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Delta also seems particularly attuned at cracking into cells to start an infection. Once it has a toehold, it starts cranking out copies of itself like the viral version of the brooms in “The Sorcerer’s Apprentice,” which can then go on to infect neighboring cells, quickly driving the viral load up beyond what other variants can do and outpacing the body’s initial response.

“All it takes is to get into a few cells to start a cascade of infection events,” Kamil said.

As the virus gets into more cells, it can bring on symptoms, which is why you’ve likely heard from friends or family about how they’ve felt sick for a few days after getting a breakthrough infection. But soon after the infection onset, the immune system — trained by the vaccine (or an earlier infection) to recognize and fight off the coronavirus — brings in the big guns, with an onslaught of additional antibodies and immune cells that can clear out the virus before it leads to more serious consequences. This is why, even if Delta is causing infections in some people who are vaccinated, in nearly all cases, it is causing only up to moderate disease — the goal the shots were designed to achieve.

There are other factors outside of Delta’s traits that are likely contributing to the increase in breakthrough infections, experts caution. For one, mitigation efforts have largely lapsed and people have restarted their social lives, leaving us more likely to come into contact with the virus.

Beyond that, it was always going to be difficult to completely ward off a pathogen that first infects the upper airway. The strong immune response generated by the vaccines that is so powerfully protecting people from severe Covid-19 is much harder to generate and sustain in the nose and throat.

“It’s a very tall order to ask a shot in your arm to protect the cells in your nose or in your mouth,” said immunologist Michal Tal, who has affiliations at Stanford and MIT.

What’s more, the antibodies generated by the vaccines, which are the body’s front line fighters in trying to block infections, wane months out from getting immunized. This is what happens with antibodies generally, and does not mean that people are more susceptible to getting severe disease. But it does mean that people could be more vulnerable to an infection months out from their last dose.

“The combination of not as much antibody being there, and the virus so quickly getting into cells,” helps explain the increase in breakthrough infections, said longtime coronavirus researcher Stanley Perlman of the University of Iowa.

It’s a different story in people who are unvaccinated and don’t have immune protection. And some research has indicated that, among these people, Delta is more likely to cause disease severe enough to put them in the hospital than earlier forms of the virus — though the evidence is divided.

A study out of the United Kingdom, for example, found people infected with Delta faced twice the risk of hospital admission compared to people infected with the Alpha variant, which was echoed by research out of Denmark. A study out of Norway, however, found no such difference, and a report from the Centers for Disease Control and Prevention last week noted that “it is not clear whether the Delta variant causes more severe illness in adult or pediatric populations.”

Researchers note it’s hard to compare disease severity from one point in the pandemic to another and from one variant to another, given all the variables involved: the demographics of people getting infected, the burden on the health system, improvements in care over time, and more. And many experts argue that the cases overwhelming hospitals in some states are just a reflection of the uncontrolled spread of the virus, not anything particular about this version.

But if Delta does cause severe disease more frequently than other forms of the virus, one possible explanation is how quickly its levels explode, experts said. If there are such high levels of virus in the nose and throat, the thinking goes, then it’s more likely that the virus will be able to infiltrate into the lungs, which is when more serious consequences can occur.

“If you have much more virus in the nose, why wouldn’t you have much more virus in the lungs as well?” Perlman said.

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