Decreasing median age of COVID-19 cases in the United States: changing epidemiology or changing surveillance?

Greene, Dina N; Jackson, Michael L; Hillyard, David R; Delgado, Julio C; Schmidt, Robert L

Background Understanding and monitoring the demographics of SARS CoV2 infection can inform strategies for prevention. Surveillance monitoring has suggested that the age distribution of people infected with SARS CoV2 has changed since the pandemic began, but no formal analysis has been performed. Methods Retrospective review of SARS CoV2 molecular testing results from a national reference laboratory was performed. Result distributions by age and positivity were compared between early period (March to April 2020) and late periods (June to July 2020) of the COVID 19 pandemic. Additionally, a subanalysis compared changing age distributions between inpatients and outpatients. Results There were 277,601 test results of which 19320 (7.0%) were positive. The median age of infected people declined over time (p < 0.0005). In March-April, the median age of positive people was 40.8 years (Interquartile range (IQR): 29.0, 54.1). In June-July, the median age of positive people was 35.8 years (IQR: 24.0, 50.2). The positivity rate of patients under 50 increased from 6.0 to 10.6 percent and the positivity rate for those over 50 decreased from 6.3 to 5.0 percent between the early and late periods. The trend was only observed for outpatient populations. Conclusions We confirm that there is a trend toward decreasing age among persons with laboratory-confirmed SARS CoV2 infection, but that these trends seem to be specific to the outpatient population. Overall, this suggests that observed age-related trends are driven by changes in testing patterns rather than true changes in the epidemiology of SARS CoV2 infection. This calls for caution in interpretation of routine surveillance data until testing patterns stabilize.