CDC: Delta as contagious as chickenpox

Mask mandate sign
Mask mandate sign

Chad Davis / Flickr cc

The Centers for Disease Control and Prevention (CDC) say the Delta variant (B1617.2) is as contagious as chickenpox in an internal document leaked to the Washington Post today. The agency also said that, although vaccinated people rarely get serious breakthrough infections, when they do get infected they can transmit the virus as easily as unvaccinated people.

The document offers insight as to why CDC Director Rochelle Walensky, MD, MPH, earlier this week said vaccinated Americans should once again don masks when indoors in areas of high transmission—a category that 69.3% of US counties fall under, according to Reuters. High transmission is defined as 50 new daily cases per 100,000 people during a 7-day period.

In May, the CDC had said fully vaccinated people no longer needed to wear masks.

The CDC estimates there are now 35,000 symptomatic infections per week among 162 million vaccinated Americans, according to the document.

Massachusetts outbreaks shows high viral load in vaccinated

Much of the recent decision-making on mask use was based on data gleaned from a new report in Morbidity and Mortality Weekly Report, which detailed July 2021 outbreaks in Barnstable County, Massachusetts. Thousands of tourists traveled to the coastal Cape Cod towns in the county during the week following the Fourth of July.

A total of 469 COVID-19 cases were identified among Massachusetts residents who had traveled to the town from Jul 3 to 17; 346 (74%) occurred in fully vaccinated persons, the CDC said. Seventy-nine percent of patients with breakthrough infections were symptomatic.

Testing showed 90% of specimens from 133 patients were the Delta variant. Four of the vaccinated patients were hospitalized, and as of Jul 27, no deaths were reported.

"Real-time reverse transcription–polymerase chain reaction (RT-PCR) cycle threshold (Ct) values in specimens from 127 vaccinated persons with breakthrough cases were similar to those from 84 persons who were unvaccinated, not fully vaccinated, or whose vaccination status was unknown," the authors said, meaning viral loads were the same in both vaccinated and unvaccinated people.

In a press statement, Walensky said the data from Massachusetts was showing the Delta behaved differently than other variants of the virus.

"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. This finding is concerning and was a pivotal discovery leading to CDC’s updated mask recommendation," Walensky said. 

"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 faces communication challenge

Throughout the internal document, the CDC said it is now facing the challenge of promoting vaccines in the face of breakthrough infections. Though the vaccines are still 90% effective in preventing severe illness against the Delta variant, they are likely less effective in preventing infection or transmission.

The agency said with current national vaccination coverage, non-pharmaceutical interventions, including masks, are needed.

In a section called "next steps for the CDC," the agency said it must "acknowledge the war has changed." The CDC said it must educate the public about the risk of breakthrough infections, and explain vaccination results in a 10-fold or greater reduction in the risk of severe disease or death, and a 3-fold reduction in the risk of infection.

The CDC COVID Data Tracker shows 397,464,625 COVID-19 vaccine doses have been delivered in the United States, and 344,071,595 have been administered. Almost half (49.4%) of Americans are fully vaccinated, and 57.2% have received at least one dose.

Yesterday the United States reported 78,170 new COVID-19 cases and 321 deaths, according to the Johns Hopkins COVID-19 tracker. The 7-day average of new daily COVID-19 cases is 71,621, according to the New York Times tracker.

Other US developments

  • The Pentagon said last night that it would require military personnel to attest to their COVID-19 vaccination status or face regular testing and other restrictions, according to the New York Times.

  • Texas Gov. Greg Abbott issued an executive order yesterday reiterating his opposition to mask mandates, COVID-related restrictions, and vaccine requirements, according to CNBC. Hospitalizations and the state's positivity rate have climbed to levels not seen since the spring.

  • Starting in August, West Virginia will begin measuring the level of antibodies in vaccinated nursing home residents to determine whether they might need booster shots, Kaiser Health News reports. 

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