Before revision:  Second, we discuss how to add standard matched case-control studies (separately for SARS-CoV-2 test-positives and  test-negatives ) by asking accompanying persons to the test facilities to become controls. Those case-control studies give a contrast between COVID-19 patients and matched persons from the general population. We give suggestions for other types of population controls .
After revision:  Second, we discuss how to add standard matched case-control studies (separately for SARS-CoV-2 test-positives and test-negatives ) by asking accompanying persons to the test facilities , the other drawn from existing country-wide health care databases. We also describe other types of population controls .