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Transmission Dynamics by Age Group in COVID-19 Hotspot Counties – United States, April–September 2020

Our take —

This ecological study showed consistent trends in percent positivity by age group across US regions inclusive of 767 hotspot counties, with early increases in the percent positivity among persons aged <25 years, prior to the identification as a hotspot, followed by several weeks of increasing percent positivity in persons aged 25 years or more. The findings indicate the importance of reducing transmission from younger populations to older ones, and suggest that increasing youth percent positivity can warn of impending increases in COVID-19 cases and hospitalizations among older populations.

Study design

Ecological

Study population and setting

This study analyzed temporal trends in SARS-CoV-2 testing percent positivity by age group in 767 COVID-19 hotspot counties in the US categorized by geographic region, before and after their identification as hotspots. Hotspots were counties that had >100 cases during the past 7 days and experienced increases in cases in the preceding 3–7 days. SARS-CoV-2 RT-PCR test results were obtained from state health departments and laboratories. Percent positivity was calculated by age group (0–17, 18–24, 25–44, 45–64, and ≥65 years) for the 45 days before and 45 days after hotspot detection, spanning April to September 2020.

Summary of Main Findings

Among 767 hotspot counties identified during June and July 2020, particularly those in the South and West, early increases in the percent positivity among persons aged ≤24 years were followed by several weeks of increasing percent positivity in persons aged ≥25 years. Percent positivity among persons aged 0–17 and 18–24 years began increasing 31 days before hotspot identification. Increases in percent positivity among older age groups began after the increases in younger age groups: among adults aged 25–44 years, 45–64 years, and ≥65 years, increases began 28 days, 23 days, and 20 days, respectively, before hotspot identification. Percent positivity among persons aged 18–24 years was near its peak of 15% by the date of hotspot detection; however, among other age groups, percent positivity continued to increase for 21 to 33 days after hotspot detection, peaking at 10 to 14%, and the decline for other age groups was slower than that for persons aged 18–24 years.

Study Strengths

The study included a large sample: 767 counties representing 24% of all US counties and 63% of the US population, and likely included near-complete test data from those counties. The study analyzes four geographic regions, allowing for the identification of regional trends.

Limitations

The numbers of tests included in the analyses are not provided.

Value added

The study provides evidence that persons aged 18–24 years demonstrate the earliest increases in percent positivity, suggesting that increases in that age group may be a harbinger of percent positivity increases for other age groups, and underscoring the importance of reducing transmission from younger populations to those at highest risk for severe illness or death. It demonstrates the urgency of health care preparedness in hotspot counties, which are likely to experience increases in COVID-19 cases and hospitalizations among older populations in the weeks after meeting hotspot criteria.

This review was posted on: 9 December 2020