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It’s getting even riskier to remain unvaccinated.

The United States, as a whole, is still in good shape for the summer of reunions and revived activities. But for those who haven’t been immunized against Covid-19, there is a new concern: the emergence of yet another coronavirus variant, one with a nasty combination of features that makes it even more dangerous than the other strains that have caused global alarms.

The variant, known as Delta, was first spotted in India and helped power that country’s recent explosive outbreaks. Also called B.1.617.2, it seems to be the most transmissible version of the coronavirus seen thus far, but also carries some ability to get around the body’s immune protection generated after vaccination or an initial infection. (There’s also some evidence that it is more likely to cause severe disease, though researchers are still trying to confirm that.)

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“It’s really like the worst of both worlds,” said epidemiologist Nathan Grubaugh of the Yale School of Public Health.

Globally, the variant’s march around the world could ignite major epidemics, given vaccine shortages in many nations. In unvaccinated populations, experts generally have greater fears about more transmissible variants than ones that are, say, just deadlier, because by causing more cases than would have occurred otherwise, the faster spreading strains can result in greater hospitalizations and deaths overall. Delta appears to be able to do that and more.

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“Taken all together, this is something which is really, really anxiety-inducing from a global health perspective,” said epidemiologist William Hanage of Harvard’s T.H. Chan School of Public Health.

In the United States, Delta accounts for just 6% of sequenced cases, federal health officials said this week, though its prevalence is building.

On an individual level, people who have had their full vaccine regimens — a figure that this week in the U.S. crossed 50% among those 12 and older — don’t need to be particularly worried about Delta. The Covid-19 immunizations retain the large bulk of their effectiveness against the variant, though with different vaccines used in different parts of the world, scientists are still parsing how each of them fare. Experts so far have been particularly impressed with how strongly the two-dose mRNA vaccines from Moderna and Pfizer-BioNTech, which account for the bulk of shots delivered in the U.S., stand up to different versions of the virus.

But it’s not as if the SARS-CoV-2 virus has disappeared from the United States. It’s been clear that it would continue to circulate among unvaccinated people going forward. Now, any clusters that emerge and are driven by Delta will grow bigger and faster. And for those who do contract it — with some evidence from England indicating that people infected with Delta are more likely to be hospitalized —“it’s going to be worse for them,” Hanage said.

With so many people vaccinated, whatever outbreaks occur likely won’t lead to huge spikes that inundate hospitals — but they will lead to some people dying.

Nationwide, cases have plunged from where they were just months ago, though the pace of decline has slowed. Study after study has shown the vaccines aren’t just protecting people from Covid-19, but are blunting transmission.

That wall of protection will benefit people who are unvaccinated as well, as will other factors that could be contributing to lower transmission rates, including immunity levels from natural infection and seasonal factors that seem to put some brake on spread.

But some unvaccinated people remain more vulnerable than others.

The story of the pandemic — both globally and within the United States — has been one of hot spots and areas that are better protected. Now, states where immunization rates are lagging — like Idaho, Wyoming, Louisiana, Alabama, Tennessee, and Mississippi — are going to be more susceptible to ongoing outbreaks than states like those in the Northeast, where vaccine uptake has been much higher, particularly as communities slough off remaining countermeasures.

Plus, more transmissible variants like Delta require a higher level of population immunity to be slowed.

The silver lining of the appearance of different variants is that, for whatever twists evolution has thrown our way, the vaccines have largely been able to withstand them. Variants that have some capability to “escape” the immune response elicited by vaccines might cause breakthrough infections at higher rates, and their transmission might not be slowed as quickly. The shots, however, have maintained their incredibly robust ability to prevent sickness and severe outcomes from Covid-19.

But research into Delta has also underscored the importance of getting both shots of the two-dose vaccines. A study from Public Health England last month showed the Pfizer regimen was 88% effective at protecting against symptomatic illness from the variant, and the AstraZeneca vaccine was 60% effective  — just slight drops in performance compared to other forms of the virus. But after just one shot, those figures plummeted to 33% for both vaccines. (The AstraZeneca vaccine has not been authorized in the United States.)

“After the second dose, we see the amount of antibody in a person’s blood is even higher than after the first dose, and therefore more people will be above a threshold,” Wendy Barclay, a virologist at Imperial College London, said at a press briefing Wednesday. “There’s a certain level of antibody and immunity you need to be protected. We know that’s a certain level for the Alpha variant, and there will be a slightly higher level required to protect against Delta variant.”

The Alpha variant Barclay referenced is also known as B.1.1.7 and first appeared last fall in the United Kingdom. Before Delta, it was the most transmissible variant and became dominant in a number of countries, including the United States, but does not have any meaningful immune escape prowess.

Scientists have estimated that Delta is perhaps 60% even more transmissible than Alpha. And in the U.K., it’s overtaken Alpha in frequency — which some experts have speculated could happen in the United States as well. The latest U.S. data indicate Alpha accounted for nearly 70% of sequenced cases, but Delta is amassing.

The urgency, then, is to get as many people immunized as quickly as possible, so more people are protected and there are fewer infections overall, as Delta increasingly accounts for more of the country’s cases.

“If you’ve had your first dose, make sure you get that second dose, and for those who have been not vaccinated yet, please, get vaccinated,” Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, said at a White House briefing Tuesday, where he highlighted the threat of Delta.

There are several factors that appear to give Delta its transmission boost over Alpha. For one, Delta “can transmit more easily in a vaccinated population than the Alpha variant can,” epidemiologist Neil Ferguson of Imperial College London told reporters Wednesday. That advantage will become particularly noticeable in countries with higher vaccination coverage, like the United Kingdom and the United States, because Alpha will encounter greater barriers as it tries to spread. (It’s not that Delta can run rampant through vaccinated populations; it’s a matter that it can transmit comparatively more efficiently because of its immune escape capability.)

In fact, some experts anticipate that in the United States, variants like Delta, Beta, and Gamma could start to claw away at Alpha’s dominance because they have greater abilities to spread among vaccinated populations, though the likelihood of that will depend on how many people remain unvaccinated. (Beta, or B.1.351, first appeared in South Africa, while Gamma, or P.1, emerged in Brazil.)

But Delta also appears to have some other advantages. “The virus itself is fitter in human airway cells,” Barclay said about what early research indicates. That means that an infected person will likely emit more virus into the air, and that the virus is more adept at infecting cells, so people need to be exposed to less of it to contract it.

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