Delta, now in at least 132 countries and already the dominant form of the disease in the United States, is more transmissible than the common cold, the 1918 Spanish flu, smallpox, Ebola, MERS and SARS, according to the document. Only measles appears to spread faster than the variant.
The data published Friday was based on 469 cases of Covid associated with multiple summer events and large public gatherings held in July in Barnstable County, Massachusetts, which encompasses Cape Cod and is just outside Martha’s Vineyard. The events were held in Provincetown, according to NBC News. Approximately three-quarters, or 74%, of the cases occurred in fully vaccinated people who had completed a two-dose course of the mRNA vaccines or received a single shot of Johnson Johnson’s.
Overall, 274 vaccinated patients with a breakthrough infection were symptomatic, according to the CDC. The most common side effects were cough, headache, sore throat, muscle pain and fever. Among five Covid patients who were hospitalized, four were fully vaccinated, according to the agency. No deaths were reported.
Testing identified the delta variant in 90% of specimens from 133 patients.
The CDC said the data has limitations. The agency noted that as population-level vaccination coverage increases, vaccinated persons are likely to represent a larger proportion of Covid cases. Additionally, asymptomatic breakthrough infections might be underrepresented because of detection bias, the agency said.
The CDC also said the report is “insufficient” to draw conclusions about the effectiveness of the authorized vaccines against Covid, including the delta variant, during this outbreak.