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Not too long ago, the end game of the coronavirus pandemic seemed straightforward. After the nation experienced horrific surges of cases, hospitalizations, and deaths in 2020 and early in 2021, a campaign to administer highly effective vaccines appeared to be bearing fruit. Restrictions were loosened. And case numbers dropped all the way into July, offering the golden prospect of a much-anticipated “summer of freedom.”

At a large July 4 celebration at the White House, President Biden said, “Thanks to our heroic vaccine effort, we’ve gained the upper hand against the virus,” and “we are closer than ever to declaring our independence from a deadly virus,” though he also acknowledged that challenges remained, including the Delta variant. People across the nation were dropping their masks and breathing a sigh of relief.

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But behind the scenes the supercontagious variant was on the move.

By the end of July it was accounting for more than 9 out of 10 new cases. And it has been propelling a surge in cases, hospitalizations, and deaths. With more than 100,000 cases a day on average nationally, the US Centers for Disease Control says the nation is seeing “high” community transmission of the virus. Places where vaccination rates were lagging have been hit especially hard.

As the threat from Delta has come into focus, there’s been a flurry of activity in response. The CDC has changed its guidance on indoor mask-wearing for people who are vaccinated, and some communities have reimposed indoor masking mandates. President Biden instituted tough new vaccination rules for federal employees, and companies and universities have begun requiring vaccines. People are also looking into whether they should get booster shots to protect themselves.

Overall case, hospitalization, and death counts remain lower than in previous surges, thanks to the vaccines. However, our knowledge of the Delta variant is evolving and in the process changing the way we view the pandemic.

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An internal CDC document leaked in late July said it all. The report called for officials to acknowledge that “the war has changed.”

Here’s what we’ve learned:

Delta is more contagious than the common cold

The CDC estimates that the Delta variant is nearly twice as infectious as previous variants. The leaked CDC document put it in simple terms: It said the Delta variant is more transmissible than the flu and the common cold (both of which can seemingly come out of nowhere). It also said the virus is about as transmissible as chickenpox and is only slightly less contagious than measles, which is considered one of the most transmissible viruses.

Dr. Rochelle Walensky, the CDC director, told CNN after the document was revealed, “I think people need to understand that we’re not crying wolf here. This is serious. It’s one of the most transmissible viruses we know about. Measles, chickenpox, this -- they’re all up there.”

Dr. Larry Brilliant, a prominent epidemiologist, told CNBC Friday, “This is maybe the most contagious virus that we’ve ever seen in living memory.”

So now we have to get even more people vaccinated

The increased transmissibility has changed the calculus for what is needed to end the pandemic.

Epidemiologists had hoped that getting 70 or 80 percent of people vaccinated, combined with the number of people who had developed some immunity from being previously sick with the virus, would control the pandemic.

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But the increased transmissibility of Delta means the target for vaccinations has to be higher, perhaps in the 90 percent range.

Empty vials of the Pfizer-BioNTech COVID-19 vaccine.
Empty vials of the Pfizer-BioNTech COVID-19 vaccine.Matthias Schrader/AP/File

Delta is believed to spread from vaccinated people

While vaccines are highly effective at blocking severe disease from Delta, officials say people can still get less serious cases and spread it, something which was not a major concern previously. That was another game-changer.

The CDC, citing a study of an outbreak in Provincetown, among other data, said similar viral loads were found in both vaccinated and unvaccinated people. “High viral loads suggest an increased risk of transmission and raised concern that, unlike with other variants, vaccinated people infected with Delta can transmit the virus,” Walensky said in a statement. Vaccinated people could, she said, “unknowingly transmit virus to others, including their unvaccinated or immunocompromised loved ones.”

Walensky said the fact that vaccinated people can spread the virus was a “pivotal discovery” that led in late July to the agency advising vaccinated people to put their masks back on indoors. The agency now recommends that all people, vaccinated or not, wear masks in public indoor settings in area with “substantial” or “high” coronavirus transmission. That now includes most of the country, except for a few fortunate spots.

People gathered at City Hall to protest vaccine mandates on Monday in New York City.
People gathered at City Hall to protest vaccine mandates on Monday in New York City. Michael M. Santiago/Getty Images

The ‘pandemic of the unvaccinated’

During the heady days of June, it may have seemed that the entire nation was about to ride a wave of vaccinations and defeat the virus together. But what’s emerged since, with the rise of a tenacious Delta, is a tale of two countries, one in which people are vaccinated and protected from the virus, and another in which people are unvaccinated and potentially prey to it.

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The officials say the country is in the midst of a “pandemic of the unvaccinated.”

“If you are vaccinated, you are very well protected against getting infected. Since no vaccine is 100 percent protective, there will be what we call ‘breakthrough’ infections. That’s the bad news. The good news is that almost invariably that will be an infection that is either without symptoms or minimally symptomatic,” Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said Sunday on NBC-TV’s “Meet the Press.”

However, the Delta variant is “extremely dangerous for the unvaccinated,” he said. “If they stay unvaccinated and don’t wear masks, ultimately they’re going to wind up getting infected and that’s what we’re concerned about.”

Federal officials have suggested that as many as 99 percent of the people who have died recently from coronavirus are unvaccinated, while almost as high a percentage of people who were hospitalized were unvaccinated. Walensky said last week, however, that those numbers “didn’t reflect the data we now have from the Delta variant” and CDC was working to update the numbers.

“I do want to reiterate, though, that based on the data we’re seeing ... universally, as we look at our hospitalizations and as we look at our deaths, they are overwhelmingly unvaccinated people,” she said at a briefing of the White House COVID-19 response team.

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An intubated patient inside a negative pressure room in the Covid-19 ICU at a hospital West in Joplin, Missouri on Aug. 3.
An intubated patient inside a negative pressure room in the Covid-19 ICU at a hospital West in Joplin, Missouri on Aug. 3.Angus Mordant/Bloomberg

Could Delta be more severe?

Although it’s clear Delta is more transmissible, scientists are less certain about whether it causes more severe illness. Some data suggest the Delta variant might cause more severe illness than previous strains in unvaccinated persons,” the CDC said in a summary of the scientific evidence.

The document leaked in late July from the CDC asserts at one point that Delta “may cause more severe disease,” pointing to studies from Canada, Singapore, and Scotland. At another point, it suggests it is “likely” to be more severe.

Front-line doctors have disturbing anecdotal reports. One Louisiana doctor told The Washington Post that patients “get sicker quicker.” An Alabama doctor said that inpatient data support the idea that “delta may be more harmful.”

But Walensky said Thursday that the CDC document cited “early preliminary studies.” She noted that the Delta variant spread rapidly in a time when mitigation efforts such as masking and social distancing had been relaxed. That has led to many more people being exposed and made it difficult for researchers to disentangle the severity of the virus from important changes in how people are exposed to it.

Concern is also being raised that the variant might make children sicker than previous variants. “Everybody is a little bit nervous about the possibility that the delta variant could in fact be, in some way, more dangerous in kids,” Dr. Richard Malley, a pediatric infectious disease specialist at Boston Children’s Hospital, told The New York Times.

“We continue to monitor this not just in adults, but also in children,” Walensky said at the briefing.

One other problem caused by the rise of the Delta variant

Experts have an additional concern about the continued rise of the Delta variant. The longer it circulates, they say, the greater the chance it will develop a mutation that could have even worse characteristics.

“There’s a tenet that everybody knows in virology: A virus will not mutate unless you allow it to replicate. So if you allow the virus to freely circulate and not try and stop it, sooner or later there is a likelihood that you will get another variant that could - I’m not saying it will - that could be more problematic than the Delta,” Fauci said.

“If you give the virus a chance to continue to change, you’re leading to a vulnerability that we might get a worse variant and then that will impact not only the unvaccinated that will impact the vaccinated because that variant could evade the protection of the vaccine,” he said.

Dr. Howard Koh, a professor at the Harvard T.H. Chan School of Public Health, raised the same concern in an e-mail, warning, “Emerging variants in the future could potentially escape immunity offered by the current vaccines and cause even more societal disruption and chaos.”

Material from Globe wire services was used in this report.


Martin Finucane can be reached at martin.finucane@globe.com.