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July 30, 2021

Delta much more contagious than other COVID-19 variants, CDC document says

An internal presentation suggests that vaccinated people, when infected, spread the virus at similar rates to unvaccinated people

With COVID-19 cases rapidly rising again across much of the United States, the U.S. Centers for Disease Control and Prevention is now comparing the infectiousness of the Delta variant to chicken pox — one of the world's most transmissible viruses. 

An internal document from the CDC — a slide presentation — reportedly provides an outline of unpublished data on the Delta variant that suggests it might be spread at the same rate by unvaccinated people and fully vaccinated people who get breakthrough infections, regardless of whether they experience symptoms. It also appears to result in more serious illness. 

The document, first reported by the Washington Post, still says vaccinated people are three times less likely to contract COVID-19 and 10 times less likely to die from the virus than those who are unvaccinated.

"I think people need to understand that we're not crying wolf here. This is serious," CDC Director Dr. Rochelle Walensky told CNN, confirming the authenticity of the internal document. "It's one of the most transmissible viruses we know about. Measles, chickenpox, this – they're all up there."

In light of the newer findings, the CDC is again recommending that fully vaccinated people wear masks indoors in places where COVID-19 transmission is sustained or high.

Pennsylvania and much of the Northeast are not among the states that have seen the largest surges in COVID-19 in recent weeks, but transmission rates are clearly on the rise despite relatively higher levels of vaccination in these states.

In Pennsylvania, the state health department reported 1,110 new COVID-19 infections on Friday. The seven-day average was 815 as of Wednesday, a fivefold increase since July 4. Hospitalizations from COVID-19 also are rising in the state.

Philadelphia officials made a strong recommendation last week that vaccinated people resume wearing masks indoors in public places, citing a small but rising number of children who have become infected and required hospitalization. Children under the age of 12 are still ineligible to receive COVID-19 vaccines and are also feared to be a growing vector for the virus, especially as they prepare to return to schools in the fall.

The CDC document reportedly indicates that breakthrough cases among vaccinated people likely will begin to represent a larger proportion of overall COVID-19 cases across the U.S. One estimate suggested there currently are about 35,000 symptomatic infections per week among 162 million vaccinated Americans. That makes the number of asymptomatic cases and spread among this group an increasingly urgent concern because about half of Americans remain unvaccinated.

"The bottom line was that, in contrast to the other variants, vaccinated people, even if they didn't get sick, got infected and shed virus at similar levels as unvaccinated people who got infected," Dr. Walter Orenstein, of the Emory Vaccine Center, told CNN after viewing the CDC documents.

Among the more troubling findings of the new research is that older vaccinated people over 60 appear to be less protected from the Delta variant than previous variants — about 97% protected from the earlier Alpha variant, compared to just 85% protection from the Delta variant, according to an Israeli study.

Additional data collected by Pfizer now points to lower vaccine efficacy after six months, falling to 84% from about 96% during the first two months, the pharmaceutical giant's CEO, Albert Bourla, said this week. 

The emerging picture is beginning to point more clearly to the likely need for a COVID-19 booster shot to extend strong protection among those already vaccinated.

CDC officials expressed a need to address new communication challenges about COVID-19 vaccines. They worry many unvaccinated people will continue to avoid them due to a misunderstanding of their effectiveness in preventing serious illness.

"We wouldn't be in this pickle if we'd had 70% of the population vaccinated," said Dr. Eric Topol, director of the Scripps Research Translational Institute in California, in an interview with USA Today. "When you have more than half your population not (fully) vaccinated, you're vulnerable."

Dr. Robert Wachter, chair of the department of medicine at the University of California, San Francisco, told USA Today that the growing understanding of where the virus is headed points to an uncertain period ahead that may require a return to more serious public health measures.

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

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