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

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.