Mixed methods approach to examining the implementation experience of a phone-based health research survey investigating risk factors for SARS-CoV-2 infection in California (preprint)

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.

Predictors of SARS-CoV-2 infection following high-risk exposure (preprint)

ABSTRACT Background Non-pharmaceutical interventions (NPIs) are recommended for COVID-19 mitigation. However, the effectiveness of NPIs in preventing SARS-CoV-2 transmission remains poorly quantified. Methods We conducted a test-negative design case-control study enrolling cases (testing positive for SARS-CoV-2) and controls (testing negative) with molecular SARS-CoV-2 diagnostic test results reported to California Department of Public Health between 24 February-26 September, 2021. We used conditional logistic regression to assess predictors of case status among participants who reported contact with an individual known or suspected to have been infected with SARS-CoV-2 (“high-risk exposure”) within ≤14 days of testing. Results 643 of 1280 cases (50.2%) and 204 of 1263 controls (16.2%) reported high-risk exposures ≤14 days before testing. Adjusted odds of case status were 2.94-fold (95% confidence interval: 1.66-5.25) higher when high-risk exposures occurred with household members (vs. other contacts), 2.06-fold (1.03-4.21) higher when exposures occurred indoors (vs. not indoors), and 2.58-fold (1.50-4.49) higher when exposures lasted ≥3 hours (vs. shorter durations) among unvaccinated and partially-vaccinated individuals; excess risk associated with such exposures was mitigated among fully-vaccinated individuals. Mask usage by participants or their contacts during high-risk exposures reduced adjusted odds of case status by 48% (8-72%). Adjusted odds of case status were 68% (32-84%) and 77% (59-87%) lower for partially- and fully-vaccinated participants, respectively, than for unvaccinated participants. Benefits of mask usage were greatest when exposures lasted ≥3 hours, occurred indoors, or involved non-household contacts. Conclusions NPIs reduced the likelihood of SARS-CoV-2 infection following high-risk exposure. Vaccine effectiveness was substantial for partially and fully vaccinated persons. KEY POINTS SARS-CoV-2 infection risk was greatest for unvaccinated participants when exposures to known or suspected cases occurred indoors or lasted ≥3 hours. Face mask usage when participants were exposed to a known or suspect case reduced odds of infection by 48%.