Before revision:  We also describe other types of population controlsthat 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  .
After revision:  We also describe other types of population controlsthat 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 . This combined design can be applied to future epidemics, but also to study causes of non-epidemic disease .