![]() For COVID-19 VE studies, controls could be patients with an ARI who test-negative for SARS-CoV-2 (test-negative controls) or patients without an ARI (syndrome-negative controls). The primary purpose of control groups in VE studies is to provide vaccine coverage estimates among people without the infection of interest from the same population as case-patients. However, while theoretical comparisons between the study designs have been detailed in the literature, empirical evidence on control group comparability does not yet exist for COVID-19 VE evaluations. Evaluations of control groups between TND and traditional case-control designs for influenza and rotavirus VE studies show good comparability,. ![]() Due to these logistical and methodologic advantages, TND has been widely employed as the de facto standard for evaluating COVID-19 vaccine effectiveness during the pandemic. TND offers two main advantages over traditional case-control studies: (1) simplified enrollment of control-patients who are captured during the case identification process and (2) comparable healthcare seeking behavior between case- and control-patients as they have the same clinical syndrome. In contrast to TND, traditional case-control studies include “disease-free” control-patients without the clinical syndrome of interest (hereafter “syndrome-negative controls”). Control-patients who present with the clinical syndrome of interest (e.g., acute respiratory illness ) but are not infected with the vaccine-preventable pathogen of interest (e.g., influenza virus) are classified as “syndrome-positive, test-negative controls” (hereafter “test-negative controls”). TND combines features of both prospective cohort and case control studies as patients with a clinical syndrome are prospectively enrolled prior to knowledge of disease status, and laboratory testing is used to perform post-hoc classification of patients as cases and controls. TND is an established tool for evaluating vaccine effectiveness in influenza and rotavirus. In consideration of the unique epidemiology of COVID-19, the WHO recommends the test-negative design (TND) for use in COVID-19 VE evaluations however, methodological challenges remain with evaluation of COVID-19. Vaccine effectiveness studies also address evidence gaps on protection against illness progression that are critical for evaluating diseases such as COVID-19 with an increased risk of severe outcomes in select populations. Post-authorization evaluations of vaccine performance help to determine vaccine effectiveness (VE) in the context of clinical and implementation factors not fully assessed by licensure clinical trials such as underlying medical conditions, duration of protection, comparison of vaccine products and types, protection against emerging variants, and diverse clinical outcomes,. Since the first Emergency Use Approval (EUA) for a COVID-19 vaccine from the World Health Organization (WHO) in December 2020, there has been ongoing investigation of performance of these vaccines in real-world settings. Hart, ag Meredith McMorrow, a Diya Surie, a Wesley H. Stubblefield, ae Adrienne Baughman, ae Jillian P. Chang, z Christopher Mallow, aa Carolina Rivas, aa Jennie H. Wilson, y Alexandra June Gordon, y Nida Qadir, z Steven Y. ten Lohuis, w Abhijit Duggal, x Jennifer G. Ginde, c Tresa McNeal, b Shekhar Ghamande, b David J. Caitlin Turbyfill, a Katherine Adams, a, ⁎ Mark W.
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