Evaluation of test-negative design estimates of influenza vaccine effectiveness in the context of multiple, co-circulating, vaccine preventable respiratory viruses

Test-negative design (TND) studies are cornerstones of vaccine effectiveness (VE) monitoring for influenza. The introduction of SARS-CoV-2 and RSV vaccines complicate the analysis of this design, with control selection restriction based on other pathogen diagnosis proposed as a solution. We conducted a simulation study and secondary analysis of 2017–18 and 2018–19 TND estimates from a Southeast Michigan ambulatory population to evaluate RSV-status-based control restriction. Simulations suggest that with vaccine-preventable RSV, influenza VE could be moderately biased with RSV prevalence ≥25 % of controls. Real-world analysis showed 151 influenza-negative adults (10.4 %) had RSV detected from the enrollment nasal swab. There were minimal differences in results of adjusted models with or without RSV exclusion from control groups. Findings suggest that inclusion of RSV cases in the control group of TND studies for influenza VE, particularly where RSV is not vaccine preventable, does not currently pose a major concern for bias in VE estimates.