Over the summer months, the U.S. experienced a shift in the age groups most impacted by the novel coronavirus, with persons ages 20-29 accounting for more than 20% of all confirmed cases. In a report issued by the Centers for Disease Control and Prevention (CDC), officials noted that the shift in age, particularly in the Southern regions hit by outbreaks in June, also suggested that “younger adults likely contributed to community transmission of COVID-19.”

The data noted that in May, the median age of Americans infected with COVID-19 was 46 and that those infected with coronavirus aged 40-49 accounted for 16.4% of the country’s cases, compared to the 20-29 age bracket which made up 15.5% of the nation’s patients. By June, the 20-29 year-olds had overtaken the 40-49 bracket and made up 20.2% of cases whereas the older patients accounted for 16.0%. By July, there was a larger gap between the two age populations with 20-somethings making up 23.2% of COVID-19 cases, and the 40-49 bracket accounting for 15.2%.

"This report provides preliminary evidence that younger adults contributed to community transmission of COVID-19 to older adults," the CDC wrote. "Across the southern United States in June 2020, the increase in SARS-CoV-2 infection among younger adults preceded the increase among older adults by 4–15 days (or approximately one to three incubation periods). Similar observations have been reported by the World Health Organization."

While the health agency called for further investigation into community transmission dynamics, it also urged "strict adherence to community mitigation strategies" and "personal preventative behaviors by younger adults" in an effort to reduce infection and transmissions to people most at risk for severe illness.

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The report was issued on the same day that CDC Director Dr. Robert Redfield appeared before a Senate committee hearing and said that an ongoing study suggests that "most Americans have not been infected with the virus," and "are still vulnerable to the infection, serious illness and death."

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Redfield said the agency was efforting large-scale antibody testing across the U.S. to better understand the "proportion of the population" that has been infected with COVID-19. He said the health agency hopes to post the analysis from the first round of the study over the next several weeks.

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As of Thursday, the U.S. had tallied more than 6.9 million cases of the novel coronavirus and over 202,000 deaths. While a number of reopening approaches have focused on testing and contact tracing, the CDC recently issued a troubling report that suggests nearly 50% of coronavirus patients did not report their close contacts.

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"The relatively low participation and cooperation with contact tracing suggest a lack of community support and engagement with contact tracing,” according to the report.

Fox News' Kayla Rivas contributed to this report.