The Delta variant of COVID-19 is definitely more transmissible than previous versions of SARS-CoV-2, but it does not appear to be more severe, according to a new study.

The CDC reports that analysis of COVID-NET data from 14 states found no significant increases in the proportion of hospitalized COVID-19 patients with severe outcomes during the time the Delta variant has been predominant.

On the other hand, the Morbidity & Mortality Weekly Report article notes that the proportion of hospitalized unvaccinated COVID-19 patients aged 18 to 49 years significantly increased during that time period.

"Lower vaccination coverage in adults aged 18–49 years likely contributed to the increase in hospitalized patients during the Delta period. COVID-19 vaccination is critical for all eligible adults, including adults aged <50 years who have relatively low vaccination rates compared with older adults," the authors point out.

The MMWR article recounts that in mid-June 2021, B.1.671.2 (Delta) became the predominant variant of SARS-CoV-2 circulating in the United States. In fact, by July 2021, the Delta variant was responsible for nearly all new SARS-CoV-2 infections in the U.S.

The researchers used data from the CDC COVID-19ÐAssociated Hospitalization Surveillance Network, a population-based surveillance system for COVID-19Ðassociated hospitalizations, to zero in on trends in severe outcomes in adults hospitalized with laboratory-confirmed COVID-19 during periods before (January-June 2021) and during (July-August 2021) Delta-variant predominance.

The study determined that COVID-19–associated hospitalization rates among all adults declined from January to June 2021 (the pre-Delta period) but jumped during July to August 2021 (the Delta period).

"Among sampled nonpregnant hospitalized COVID-19 patients with completed medical record abstraction and a discharge disposition during the pre-Delta period, the proportion of patients who were admitted to an ICU, received invasive mechanical ventilation, or died while hospitalized did not significantly change from the pre-Delta period to the Delta period," the authors note.

At the same time, however, the proportion of hospitalized COVID-19 patients who were aged 18 to 49 years significantly increased, from 24.7% (95% CI, 23.2%-26.3%) of all hospitalizations in the pre-Delta period to 35.8% (95% CI, 32.1%-39.5%, P <.01) during the Delta period. Most (71.8%) COVID-19–associated hospitalizations during the Delta period were in unvaccinated adults.

Despite earlier trends of older patients, young adults aged 18 to 49 years accounted for 43.6% (95% CI, 39.1%-48.2%) of all hospitalizations among unvaccinated adults during the Delta period. "Lower vaccination coverage in this age group likely contributed to the increase in hospitalized patients during the Delta period. COVID-19 vaccination is critical for all eligible adults, including those aged <50 years who have relatively low vaccination rates compared with older adults," the CDC notes.

Although all age groups included in this study were eligible to receive COVID-19 vaccines, as of August 31, 2021, 81.7% of adults aged 65 years and older had been fully vaccinated compared with 58.5 of adults aged 18 to 64 years. "Differences in vaccination coverage between age groups possibly contributed to the shift in proportional age distribution of hospitalized patients during the period of Delta predominance," the researchers suggest.

The authors advise that the findings in adults are similar to previous analyses of children and adolescents, which showed no significant differences in severe in-hospital outcomes between the pre-Delta and Delta periods.

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