Delta variant substantially more contagious than other variants, CDC presentation warns

Evidence is mounting about the dangers of the delta variant and how mask-wearing is essential to bring it under control, according to a government slideshow dated Thursday.

The presentation, compiled by a doctor with the Centers for Disease Control and Prevention, lays out what is known about the delta variant, which now accounts for the vast majority of COVID-19 cases in the United States.

Earlier this week, the CDC recommended that people in COVID-19 hotspots resume mask-wearing in indoor public spaces.

The agency had been criticized for making the recommendation without adequately supporting its reasons for the policy shift. But the presentation, first made public by The Washington Post, appears to do just that.

CDC declined to comment late Thursday on the slideshow or its contents.

The presentation shows that those who are vaccinated are three times less likely to catch COVID-19 and 10-times less likely to die from it than those who are unvaccinated.

Once infected, though, vaccinated people are just as contagious as those without that protection — meaning they need to wear a mask and keep their distance to avoid passing on the virus.

"The data makes a pretty compelling justification for why we need to go back to mask wearing and other public health measures," said Dr. Charles Chiu, an infectious disease specialist at the University of California, San Francisco. "I do think it's because of the delta variant."

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Mapping CDC's new guidelines: High transmission areas where you need to wear a mask indoorsThe vaccines still work extremely well and are an essential part of eventually winning the battle against COVID-19, Chiu and other experts emphasized.

If more people had been vaccinated earlier this year, cases would not be rising now and a return to masking wouldn't be necessary, said Dr. Eric Topol, director and founder of the Scripps Research Translational Institute in La Jolla, California.

"We wouldn't be in this pickle if we'd had 70% of the population vaccinated," he said. "When you have more than half your population not (fully) vaccinated, you're vulnerable."

People who are vaccinated should wear a mask and socially distance, he and others said, largely to protect those who haven't been vaccinated or who have a weakened immune system and so can't get full protection from the vaccine.

Vaccines remain effective and preventing hospitalization and death from COVID-19, though they worked better against the original strain and the alpha variant than they do against delta, data finds.

In one Israeli study, vaccinated people over 60 were shown to be 97% protected against the alpha variant, but only 85% protected against delta. "Ninety-seven percent is phenomenal, but 81%?" Topol said. "We've got a bigger challenge than ever before."

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The delta variant is known to be substantially more contagious than other variants – as contagious though deadlier than chicken pox, according to the CDC presentation. Among common infectious diseases, only measles is more contagious.

People may also be infectious for longer with the delta variant, 18 days instead of 13, the presentation says. This suggests official guidance on quarantining when sick may need to be changed, too, said Dr. Robert Wachter, chair of the department of medicine at the University of California, San Francisco.

"None of it is good news," he said by email. "Clearly (we) need more vaccinations, resumption of non-pharmacologic interventions ASAP even for vaccinated people, and probably boosters."

The federal government has said there is not yet enough data to support booster shots, even for the elderly and immunocompromised, though Israel is now recommending them for those groups.

In a study posted online this week by Pfizer and its collaborator BioNTech, but not yet peer-reviewed, the companies found their vaccine to be effective for at least six months. Effectiveness topped 90% for that period, according to the study, which was largely conducted before the delta variant became widespread.

Data beyond six months isn't yet available because the shots are so new.

Several researchers questioned why the information in the CDC's presentation was not released two days earlier, when the agency made its new mask recommendations.

In a Tuesday briefing, CDC Director Dr. Rochelle Walensky downplayed the possibility that a vaccinated person could pass on the virus, without giving the details about the viral load contained in the slideshow.

Dr. Leana Wen, an emergency physician and former Baltimore health commissioner, said the slideshow suggests CDC is worried about the risks of delta to the vaccinated, but that's not what Walensky focused on during her Tuesday briefing, Wen said.

"I don't know what to make of it, honestly," she said after seeing the slideshow. "Why wasn't this level of data revealed to the American public two days ago?"

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And, she wondered aloud, why countries like the United Kingdom and Israel are still generating so much more COVID-19 data than the U.S.

It would be simple, for instance, to ask people as they're tested for COVID-19 whether they'd had their shots, she said — but the CDC doesn't collect the data, so it's not clear what percentage of transmissions are from the vaccinated.

Dr. Paul Offit, who directs the Vaccine Education Center at the Children's Hospital of Philadelphia, said he's very concerned about the current state of America's pandemic.

Last summer, before there was a vaccine, there were about 80,000 new COVID-19 cases reported every day and nearly 500 deaths — almost exactly the same count as today although half the population is vaccinated and many other have immunity because they've already been infected.

Why haven't things gotten better, Offit asked rhetorically. "The answer is we have enough people in the population that are susceptible – and we have a virus that is clearly more contagious."

Contact Karen Weintraub at

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This article originally appeared on USA TODAY: Delta variant more contagious than others, CDC internal memo warns