CDC says Delta variant very transmissible

Aug. 2, 2021

The SARS-CoV-2 Delta variant may be as transmissible as chickenpox and cause more severe disease than earlier strains of SARS-CoV-2. In addition, breakthrough infections in people fully vaccinated against SARS-CoV-2 may be as transmissible as infections in unvaccinated people.

Those discouraging findings were included in an internal presentation from the Centers for Disease Control and Prevention (CDC), based on recent published research findings and unpublished data. The CDC document was first reported by The Washington Post.

After the presentation surfaced, the CDC made some of the data on the Delta variant public on July 30 in its Morbidity and Mortality Weekly Report (MMWR). The study, which was a collaborative effort with the Commonwealth of Massachusetts Department of Public Health, identified 469 cases of COVID-19 among Massachusetts residents who had traveled to a town in Barnstable County, MA, where several large public events were held in July. A total of 346 (74%) of those infections occurred in fully vaccinated persons.

Overall, 274 (79%) vaccinated patients with breakthrough infection were symptomatic. Among five COVID-19 patients who were hospitalized, four were fully vaccinated; no deaths were reported.

Testing identified the Delta variant in 90% of specimens from 133 of those patients. Cycle threshold values, which indicate viral load, were similar among specimens from patients who were fully vaccinated and those who were not.

In a public statement commenting on the MMWR report, CDC Director Rochelle Wallensky, MD, MPH, said, the report led to the agency’s decision to revise its guidance on mask wearing. “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 guidance, which was released last week, the CDC said fully vaccinated people should wear a mask in public indoor places in geographic locations with high rates of transmission. The agency also said that fully vaccinated people should get tested 3-5 days after exposure to someone with COVID-19, even if they don’t have symptoms.

In the unpublished presentation, the agency included evidence to support the idea that the Delta variant causes more severe disease than earlier variants, saying that the percentage of hospitalized people with COVID-19 who are vaccinated has been rising steadily, going from a nearly imperceptible level in January to 9% by May.

Wallensky said the CDC plans to release additional study results to the public as they become available.

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