Leaked CDC document, new report warn of delta’s contagiousness
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This week, as she announced stiffer mask guidance for vaccinated Americans, CDC Director Rochelle P. Walensky, MD, MPH, said she had seen data indicating that breakthrough infections caused by the delta variant can be transmitted.
Those data were made public in a leaked internal CDC document and a new report published Friday in MMWR, which warned of the delta variant’s contagiousness, including among people fully vaccinated against COVID-19.
The leaked document, which was obtained by the Washington Post and New York Times, says the delta variant of SARS-CoV-2 is “as transmissible as chicken pox” and notes in a slide labeled “Next steps for CDC” that the agency must “acknowledge that the war has changed.”
The CDC confirmed to Healio that the document was authentic.
Another slide notes that breakthrough cases caused by the delta variant “may be as transmissible as unvaccinated cases.” The data supporting that assertion, according to the slide, are from two sources: passive U.S. surveillance showing a 10-fold increase in viral load for people infected with the delta variant, and the new MMWR report, which noted no difference in mean cycle threshold values in specimens collected from vaccinated and unvaccinated people involved in an outbreak of infections associated with pubic events in Barnstable County, Massachusetts, this month.
“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 reiterated in a new statement issued Friday. “This finding is concerning and was a pivotal discovery leading to CDC’s 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.”
In the MMWR report, Catherine M. Brown, DVM, an epidemiologist for the Massachusetts Department of Public Health, and colleagues summarized 469 cases of COVID-19 that occurred in Barnstable County, where 69% of the population is fully vaccinated.
Of the 469 cases, around 90% were caused by delta and 74% occurred in individuals who were fully vaccinated. Among the fully vaccinated with an infection, 79% were symptomatic.
Five of the people infected were hospitalized, including four who were fully vaccinated. No deaths were reported.
The authors of the report noted several of its limitations, including that the data “are insufficient to draw conclusions about the effectiveness of COVID-19 vaccines against SARS-CoV-2, including the delta variant, during this outbreak.”
“As population-level vaccination coverage increases, vaccinated persons are likely to represent a larger proportion of COVID-19 cases,” the authors wrote.
They also noted that asymptomatic breakthrough infections might be underrepresented because of detection bias.
The leaked CDC document also notes the rarity of breakthrough infections, and that vaccination reduces the risk for severe disease or death by at least 10 fold and the risk for infection among vaccinated people by three fold.
Evidence that the delta variant may cause more severe disease than other strains of SARS-CoV-2 also was included in the leaked document:
- In Canada, it is associated with higher odds of hospitalization (adjusted OR [aOR] = 2.2; 95% CI 1.93-2.53), ICU admission (aOR = 3.87, 95% CI 2.98-4.99), and death (aOR = 2.37; 95% CI 1.5-3.3).
- In Singapore, it is associated with higher odds of oxygen requirement, ICU admission, or death (aOR = 4.9; 95% CI 1.43-30.78) and pneumonia (aOR = 1.88; 95% CI 0.95-3.76).
- Scotland: Higher odds of hospitalization (HR 1.85 [CI 1.39-2.47]).
References:
Abutaleb Y, et al Washington Post. ‘The war has changed’: Internal CDC document urges new messaging, warns delta infections likely more severe. https://www.washingtonpost.com/health/2021/07/29/cdc-mask-guidance/. Accessed July 30, 2021.
Brown CM, et al. MMWR Morb Mortal Wkly Rep. 2021;doi:10.15585/mmwr.mm7031e2.
Mandavilli A. New York Times. CDC internal report calls delta variant as contagious as chickenpox. https://www.nytimes.com/2021/07/30/health/covid-cdc-delta-masks.html. Accessed July 30, 2021.
Washington Post. Read: Internal CDC document on breakthrough infections. https://www.washingtonpost.com/context/cdc-breakthrough-infections/94390e3a-5e45-44a5-ac40-2744e4e25f2e/. Accessed July 30, 2021.
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Abutaleb Y, et al Washington Post. ‘The war has changed’: Internal CDC document urges new messaging, warns delta infections likely more severe. https://www.washingtonpost.com/health/2021/07/29/cdc-mask-guidance/. Accessed July 30, 2021.
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