ABSTRACT
Objective: To describe the implementation of a test-negative design case-control study in California during the Coronavirus Disease 2019 (COVID-19) pandemic. Methods: Between February 24, 2021 - February 24, 2022, 34 interviewers called 38,470 SARS-CoV-2-tested Californians to enroll 1,885 cases and 1,871 controls in a 20-minute telephone survey. We estimated adjusted odds ratios for answering the phone and consenting to participate using mixed effects logistic regression. We used a web-based anonymous survey to compile interviewer experiences. Results: Cases had 1.29-fold (95% CI: 1.24-1.35) higher adjusted odds of answering the phone and 1.69-fold (1.56-1.83) higher adjusted odds of consenting to participate compared to controls. Calls placed from 4pm to 6pm had the highest adjusted odds of being answered. Interviewers who faced participants with dire need for social services or harassment experienced poor mental health. Conclusions: We suggest calling during afternoons and allocating more effort towards enrolling controls when designing a case-control study. Remaining adaptive to the dynamic needs of the team is critical to a successful study, especially in a pandemic setting.