Before revision:  We give suggestions for other types of population controls . Such studies distinguish between exposures that are risk factors for both COVID-19 and other respiratory infections, and exposures that are risk factors for just COVID-19 or just for other respiratory infections.
After revision:  We give suggestions for other types of population controls that may be most suitable in other situations. Combining the test-negative design with population controls yields a triangulation approach that distinguishes between exposures that are risk factors for both COVID-19 and other respiratory infections, and exposures that are risk factors for just COVID-19 or just for other respiratory infections.