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CDC warns: Delta more transmissible than Ebola, as contagious as chickenpox

The Delta variant is more transmissible than Ebola and as infectious as chickenpox, warns the US Centers for Disease Control and Prevention, writes MedicalBrief. “The CDC needs to make it apparent to the public that the war has changed,” the agency said.

An internal document  and accompanying slideshow from the CDC notes that the Delta variant is not only much more contagious than other known versions of the virus. It says that the Delta variant is more transmissible than Ebola or smallpox, and as contagious as chickenpox.

The finding is dismaying, writes The New York Times, but vaccines remain the one reliable shield against the virus, in whatever form it takes. The vaccines largely prevent infection, even with the Delta variant, and greatly reduce the chances of severe illness or death should infection occur.

In the US, about 97% of people hospitalised with COVID-19 are unvaccinated, according to data from the CDC. And the unvaccinated are far more likely to spread the virus to others in their communities.

Being vaccinated against COVID-19 won’t necessarily stop you from spreading the virus, says the CDC and in a statement recommends vaccinated people wear masks indoors where case counts are high or substantial. New data show that in relatively rare breakthrough cases, the viral load of vaccinated people is the same as in unvaccinated people, which means that vaccinated people can still be superspreaders.

“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,” said CDC director Dr Rochelle Walensky.

“This finding is concerning and was a pivotal discovery leading to CDCʼs updated mask recommendation. The masking recommendation was updated to ensure the vaccinated public would not unknowingly transmit virus to others, including their unvaccinated or immunocompromised loved ones.”

CDC said the vaccines remain powerfully effective against severe illness and death, and infections in vaccinated people were comparatively rare. But the revelation follows several other recent findings about the Delta variant that have upended scientistsʼ understanding of the coronavirus.

In a New York Times report, the CDC cited an outbreak in Provincetown, Massachusetts, which quickly mushroomed to 470 cases in Massachusetts alone, as of Thursday (22 July). Three-quarters of the infected had been fully immunised, and the Delta variant was found in most of the samples they analysed. Vaccinated and unvaccinated people who were infected carried high levels of the virus, the agency reported.

Before the outbreak, noted the CDC report, the 14-day average of daily case rates in the area had plummeted to zero. But in just two weeks, the 14-day case average had shot up to 177 cases per 100,000 people.

“We need more people to be vaccinated. The vaccines are still extremely effective against delta,” said Rachael Piltch-Loeb, a public health emergency preparedness and response fellow at Harvard TH Chan School of Public Health. “There is a significant difference between having a fever and dying from COVID-19.”

Another CDC report released on Friday shows that the COVID-19 vaccines are typically safe for preteens and teens. Of 8.9 million young people who got a shot, only 863 serious events were reported as possible side effects, a minuscule fraction of the vaccinations.

The Delta variant is about as contagious as chickenpox, the document noted, and universal masking may become necessary. Still, breakthrough infections overall are infrequent, according to the agency.

Concern that CDC findings will shake public faith in vaccines

The Kaiser Family Foundation reported that the rate of breakthrough cases is less than 1% among fully vaccinated people in states that keep such data.

The gathering research into the variant throws into disarray the countryʼs plans to return to offices and schools this fall, and revives difficult questions about masking, testing and other precautions that Americans had hoped were behind them.

The New York Times notes that government officials and scientists alike are worried that the findings may shake faith in the vaccines, hobbling the nationʼs immunisation campaign, should Americans infer incorrectly that the shots are not effective.

Concerned by the lagging campaign, President Biden has ordered that all federal employees be vaccinated or face weekly virus testing. Support for vaccination mandates is growing among some corporations and in some parts of the country.

The evolving research into the Delta variant has humbled scientists worldwide, who now confront fresh, previously unconsidered, questions about the virus.

They do not fully understand the circumstances that may increase the odds of a breakthrough infection, for example, nor who may be most at risk. They do not know for certain that the Delta variant causes more severe disease in the unvaccinated who become infected, although early data suggest it does.

“We spent so much time and energy trying to figure out this damn virus last year, and how it works and all the things it does,” said Dr Robert Wachter, chairman of the Department of Medicine at the University of California, San Francisco. Learning just how different the Delta variant is from the original virus is “just jarring,” he added. “The brain doesnʼt like to keep being jerked around like this.”

Monica Gandhi, a professor of medicine at the University of California, San Francisco, said more robust measurements, like examining the virus in a lab, would be needed to make the claim that vaccinated and unvaccinated people are equally infectious.

"It doesnʼt make biological sense,” she said, adding that too much of a focus on the ability of vaccines to prevent transmission, rather than hospitalisation and death, risks “creating incredible panic”.

The CDC believes that breakthrough infections will probably grow into a larger proportion of cases as vaccination rates increase. The report follows a confidential briefing that the CDC gave to Congress last Thursday where Walensky noted the number of cases is higher than at this point in the summer last year and hospitalisations are at the same level — although a vaccine was not available then.

The CDC also reiterated that the Delta variant is much more contagious than other known versions of the virus: more transmissible than Ebola or smallpox, and as contagious as chickenpox.

Even if breakthrough infections are rare, the new data suggest the vaccinated may be contributing to increases in new infections — although probably to a far lesser degree than the unvaccinated. Breakthrough infections were always anticipated, but until the Delta variant arrived, vaccinated Americans were not believed to be drivers of community spread.

“Delta is teaching us to expect the unexpected,” said John Moore, a virologist at Weill Cornell Medicine of New York. “There are aspects of what we now know that we didnʼt see coming.”

The research underscores the urgency to escalate the vaccination pace in the United States and decrease the numbers of people susceptible to severe illness. This week, the rate of vaccination in the European Union exceeded that in the United States for the first time.

About 58% of Americans aged 12 and older are fully vaccinated. The pace of vaccination has slowed to just more than 500,000 people per day, although it has begun curving slightly upward in the past couple of weeks as infections rise again.

Comprehensive vaccination appears to keep Delta at bay

In Britain, where the variant seems to be subsiding after a surge, vaccinations were rolled out by age, and a much higher proportion of people over 50 are vaccinated than in the US.

Vaccination rates are much more patchy in the United States, said Bill Hanage, an epidemiologist at the Harvard TH Chan School of Public Health. “The upshot is that what Delta does in the UK is not necessarily what itʼs going to do in places that have more very varied vaccination,” he said.

“Things are going to be worse than they would have been without the variant,” he added. “But theyʼre going to be much better than they might have been without vaccination.”

The CDC has urged local and state officials in jurisdictions with even lower levels of the virus to consider putting into effect precautions, such as masking and limiting gatherings.

Indeed, the questions now facing Americans seem nearly inexhaustible, almost insoluble. Should companies have employees return to workplaces if vaccinated people might, on occasion, spread the variant? What does this mean for shops, restaurants and schools? Are unmasked family gatherings again off the table?

A changing war and the need for humility

The New York Times said the outbreak in Provincetown sprouted after more than 60,000 revellers celebrated the Fourth of July gathering in densely packed bars, restaurants, guesthouses and rental homes, often indoors.

“Vaccines are like hip waders,” Rasmussen said. “They keep you dry if you wade through a river, but get too deep and water will start pouring in over the top. That seems to be what happened in the Massachusetts outbreak.”

Three-quarters of the state residents linked to the outbreak reported having a cough, headache, sore throat or fever — symptoms of an infection in the upper airway — and 74% had been fully immunised.

Of the five people who were hospitalised, four were fully vaccinated — one with the Pfizer-BioNTech vaccine and three with the Johnson & Johnson vaccine. Two of the vaccinated patients had underlying medical conditions. Genetic analysis of 133 cases identified the Delta variant in 119 and a closely related virus in one additional case.

Scientists warned even last year that the vaccines might not completely prevent infection or transmission. But experts did not expect that these infections would figure significantly in the fight against the virus, nor did they anticipate how quickly the Delta variant would tear through the country.

“I thought two months ago that we were over the hump,” Wachter said. In San Francisco, the most highly vaccinated big city in the country, 77% of people over age 12 are vaccinated. And yet, the hospital where he works has seen a sharp rise, from one case of Covid-19 on June 1 to 40 now. Fifteen of the patients are in intensive care.

“If getting to 70% or 75% immunity doesnʼt protect the community, I think itʼs very hard to extrapolate what happens to a place that is 30% vaccinated,” he added. “Humility may be the most important thing here.”

Evidence shows that vaccine efficacy has taken a hit from Delta, the CDC estimating that efficacy may be 75% to 85% against the new variant rather than 94% effective or more against the original COVID-19 strain.

There are 35,000 symptomatic breakthrough cases each week, the CDC revealed, with up to 15% of deaths in May being among vaccinated people. That contrasts with previous public CDC data showing deaths occur in a tiny number of vaccinated people, just 0.0005 percent.

But that may reflect that older, immune-compromised people made up a larger proportion of vaccinations earlier in the year. “The CDC needs to make it apparent to the public that the war has changed”, said the agency.

“I read a certain level of desperation in what the CDC is saying here,” said Paul Offit, a University of Pennsylvania vaccinologist. “I think theyʼre frustrated that they canʼt get more people to take a vaccine.

“You would have thought that more than 600,000 people dying would have caught their attention.”

Offit attributes the rise in cases to the Delta variant, continued resistance to vaccines, and much riskier behaviour by many Americans. “Itʼs always been a pandemic of the vaccinated. Now itʼs a pandemic of the wilfully unvaccinated,” he added.


CDC analysis on Massachusetts outbreak (Open access)


Vaccinated People May Spread the Virus, Though Rarely, CDC Reports – The New York Times


See more from MedicalBrief archives:


CDC's health sector masking guidelines were a deadly mistake


Delta variant: Not just hyper-contagious but also multiplies more rapidly inside respiratory tract


Need for full COVID vaccination to protect against Delta — Pasteur Institute


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