Estimation of the Ascertainment Bias in Covid Case Detection During the Omicron Wave

Abstract

Covid cases in the general population have been historically underreported due to a variety of reasons including limited access to PCR testing at the start of the pandemic, lack of nation-wide surveillance testing, and discouraged testing unless symptomatic. Concerns about underreporting have increased during the Omicron surge due to the expanded use of at-home rapid tests which are not required to be officially reported. For the state of Illinois, we have found that reported cases constituted only 50%-70% of the actual cases during the pre-Omicron waves (August 2020-December 2021). During the first Omicron (BA1) wave, this fraction dropped to 20-29% (i.e., only 1 in 4 to 1 in 5 cases are reported). During the ongoing second Omicron (BA2) surge, this fraction has further decreased to 12-18% (i.e., only 1 in 6 to 1 in 8 cases are reported). These estimates have important implications on understanding the extent of the Omicron surge at the state and national levels.

Extensive SARS-CoV-2 testing reveals BA.1/BA.2 asymptomatic rates and underreporting in school children

“…Omicron variant [30,52], and due to the expanded use of rapid tests, which are not officially 10 reported in some jurisdictions [30,63]. Underreporting rates may have been particularly high among children and youth, given their relatively low risk of experiencing severe outcomes [24].…”

“…COVID-19 underreporting has been a major challenge for pandemic monitoring and response planning. These difficulties have increased with the establishment of the highly transmissible Omicron variant [30,53], and due to the expanded use of rapid tests, which are not officially The household positivity (i.e., the proportion of households which reported positive test results from K-12 students) reported by each of its Forward Sortation Areas (FSAs) is shown for each RHA, where each square corresponds to a single FSA within the RHA and the color of the square represents the reported household positivity. We include only results of FSAs for which test results of students of six or more households have been reported.…”

“…The pie chart represents the number of reported positive households for each health region as a fraction of the total number of positive households in the province. The population, area, and population density of each RHA are provided in the table in the top of the figure . reported in some jurisdictions [30,64]. Underreporting rates may have been particularly high among children and youth, given their relatively low risk of experiencing severe outcomes [24].…”

“…COVID-19 underreporting has been a major challenge for pandemic monitoring and response planning. These difficulties have increased with the establishment of the highly transmissible Omicron variant [30, 53], and due to the expanded use of rapid tests, which are not officially reported in some jurisdictions [30, 64]. Underreporting rates may have been particularly high among children and youth, given their relatively low risk of experiencing severe outcomes [24].…”

“…During the pandemic, COVID-19 prevalence has been inconsistently underreported for different reasons, such as challenges in maintaining a high testing capacity [27], discouraged testing of non-symptomatic individuals [11], and many mild or asymptomatic infections, particularly in children and youth [33]. Challenges to providing accurate COVID-19 case counts have increased with the establishment of more transmissible variants, leading to faster, and at times, uncontrolled infection spread [58], with the promotion of self-testing alongside no requirement that these results be reported [30, 64], and with increased vaccination coverage, decreasing the likelihood of severe outcomes and the need to seek health care [50]. All these factors have led to inconsistent variation of case reporting over time, challenging epidemic forecasting.…”