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In some respects, the Delta variant has changed everything in the Covid-19 pandemic. In others, the same rules still apply.

Before the variant of SARS-CoV-2 began spreading rapidly in the United States, Covid-19 vaccines were drastically cutting the number of cases. They were preventing people from being infected. And vaccinated people who got infected were unlikely to infect others.

That’s all still true, even with Delta — if just to a lesser extent.

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Despite the threat of Delta, people who are vaccinated are still less likely to get Covid-19 than those who aren’t vaccinated.

But the variant appears to be causing breakthrough infections — infections in people who have been vaccinated — at higher rates than other variants, with vaccinated people also reporting higher rates of symptomatic illness. And some vaccinated people who get infected, the Centers for Disease Control and Prevention said this week, seem to carry roughly the same level of virus in their upper airways — their noses and throats — as unvaccinated people.

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That finding, combined with outbreak investigations, indicates that some vaccinated people are likely passing on the virus, and some could be as infectious as people who aren’t vaccinated. That said, vaccinated people who are contagious are likely contagious for a shorter period of time than someone who is unvaccinated, meaning on average they will pass the virus on to fewer additional people.

Even for vaccinated people, “the game has changed a little bit,” said Stephen Kissler, an epidemiologist at the Harvard T.H. Chan School of Public Health.

This week, the new findings about transmission led the CDC to advise that masks be worn in indoor public spaces, even by vaccinated people, in areas of the country seeing substantial transmission of the virus. A CDC presentation obtained by the Washington Post raised universal masking as a way to combat Delta’s transmissibility, which the agency estimated was on par with that of chickenpox. The major concern is that vaccinated people could spread the virus to unvaccinated people — including children under 12 who are not yet eligible for vaccines — or to those who might not have mounted a strong response after being vaccinated, including those who are immunocompromised.

“There was a feeling that vaccinated people were not part of transmission chains, or very minimally,” said Stephen Goldstein, a virologist at the University of Utah. “Here, they feel like vaccinated people can become part of those chains.”

Both the CDC and outside experts believe that vaccinated people are fueling just a fraction of overall Covid-19 transmission. As a result, it’s unclear how effective requiring them to wear masks will be in slowing circulation of the virus.

But the new CDC guidelines could bolster efforts by local authorities to reimpose mask rules for everyone, which could help ensure that unvaccinated people comply with the rules.

The new mask guidance is also an attempt to buy the country more time to get more people vaccinated. If communities can cut transmission even a little, and get more people vaccinated, they can protect more people from the worst outcomes. After all, even if Delta might cause mild cases of Covid-19 more frequently among people who are vaccinated than other forms of the virus, the shots are still almost universally protective (with very rare exceptions) against hospitalization and death.

Data show that in areas of the U.S. currently under siege by the virus, daily vaccination numbers have increased. A new round of vaccine mandates from governmental agencies and companies could help drive immunization rates higher.

But with stories about vaccinated people getting Covid-19 and feeling ill making news — the CDC presentation estimated 35,000 symptomatic breakthrough infections a week — it’s also raising the question of how people can get sick even though they’ve gotten their shots.

Generally, the vaccines were designed to prevent severe disease, and they are proving remarkably successful at that. Even “mild” Covid-19 can lay someone out for a few days, but without vaccines, some of those now-mild cases would have turned severe or led to death. Making subsequent infections milder and milder  — already, many are asymptomatic — is how vaccines will help turn the coronavirus from a serious threat into what is hoped to be little more than a nuisance over time.

There are also some biological reasons why some people feel sick after a breakthrough infection, particularly when they contract the Delta variant. It’s difficult to build up a robust immune response in the nose and throat, particularly when a shot is delivered into the arm. And Delta is so adept at multiplying quickly in upper airway cells that it can start causing symptoms before the immune system kicks into gear.

But the vaccine-conferred immune response is still excellent at fending off the virus before it can invade the lungs, which is when severe disease occurs. A study published Thursday also showed that, in monkeys, lower levels of antibody were needed to halt the virus in the lungs than in the upper airway — suggesting why the vaccines were better at protecting against severe disease than they were at blocking infection entirely.

Delta’s main threat remains how transmissible it is. That is what has enabled the virus to rip through communities in states with low vaccination rates like Arkansas, Florida, Nevada, and Missouri, with the spread and the consequences mostly among people who have not been vaccinated against Covid-19.

Slowing a more transmissible pathogen requires stronger defenses than those needed to curb less effective spreaders. Vaccines help build those bulwarks, but the U.S. has aided the virus in getting the upper hand — both because so many people remain unprotected and because officials largely, and many argue understandably, abandoned mitigation efforts. After a year of isolation, individuals have also returned to their old social routines, though Delta is now complicating plans to resume life, work, and school. 

Some communities with low vaccination rates are seeing their biggest surges of the pandemic right now. The fear is that if those outbreaks aren’t contained, and if more people aren’t vaccinated, the start of school in the coming weeks could add fuel to the transmission, particularly if schools aren’t opened with precautions like masking and ventilation.

“We have the least effort going into containing the spread of the virus, as well as the more transmissible virus, combining into these really disastrous scenarios,” Goldstein said.

Ultimately, Delta-ignited surges will fade, as has been shown in other countries. The Delta-driven explosion of cases in India, where the variant was first seen, has come back to nearly pre-Delta levels. After another peak, cases are now falling in the United Kingdom, which — with some exceptions — has often foreshadowed the U.S. experience throughout the pandemic. It’s hard to say why for certain — more people becoming protected through infection or vaccination, people changing their behavior, some combination of factors — but the waves have ebbed elsewhere.

Even with cases rising again in the United States, there is one relative silver lining: fewer deaths.

Delta may on average cause more severe disease than some other forms of the virus — scientists are still sorting this out — and deaths have ticked up in recent weeks. But because vaccines are still doing such an incredible job of preventing Covid-19 from becoming fatal — and because so many vulnerable people are protected, with more than four in five of seniors fully vaccinated — deaths are not rising to the same extent that they did in pre-vaccine waves.

At this point, that counts as a victory.

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