Continued Age Shift of Confirmed Positive COVID-19 Incidence Over Time to Children and Young Adults: Washington State March - August 2020

Malmgren, J.; Guo, B.; Kaplan, H.

Background: As the coronavirus (COVID-19) epidemic passed the initial infection peak in Washington State, phased re-opening lifted stay-at-home orders and restrictions leading to increased non-essential work, social activities and gathering, especially among younger persons. Methods: A longitudinal cohort analysis of Washington State Department of Health COVID-19 confirmed case age distribution 1) March-April 2020 and 2) March-August 2020 for proportional change over time using chi square tests for significance (N=13,934, N=76,032). Results: From March 1st to April 19, 2020 COVID-19 case positive age distribution shifted with a 10% decline in cases age 60 years and older and a 20% increase in age 0-19/20-39 years (chi-square = 223.10, p <.001). Number of cases over the eight-week analysis period were 0-19 years n = 515, 20-39 years n = 4078, 40-59 years n =4788, 60-79 years n = 3221, 80+ years n = 1332. After the peak (March 22, 2020), as incidence declined in older age groups, the combined percentage of cases age 0-19 and 20-39 increased from 20% to 40% of total cases. During this time testing expanded with more testing among older age groups while case positivity shifted young. Percent positive cases by age through August 2020 increased to a consistent average of 60% less than age 40 [age 0-19 increased to 19% (N = 10257), age 20-39 increased to 42% (N = 30215)]. Conclusions: An increased share of COVID-19 incidence among children (age 0-19) and young adults (age 20-39) indicates their elevated role in propagating the epidemic by creating a reservoir of disease with risk of spillovers to more vulnerable older persons and those with comorbid conditions. Media savvy and age appropriate COVID-19 messaging may increase mitigation compliance among these less vulnerable, more mobile and lower priority vaccination age groups. As vaccines become available, mitigation will continue to be a priority to reduce overall population incidence.