Confounding and Negative Control Methods in Observational Study of SARS-CoV-2 Vaccine Effectiveness: A Nationwide, Population-Based Danish Health Registry Study

Niels Obel,1,2 Matthew P Fox,3 Malte M Tetens,2 Lars Pedersen,4 Tyra Grove Krause,5 Henrik Ullum,5 Henrik Toft Sørensen4 1Department of Infectious Disease Epidemiology and Prevention, Statens Serum Institut, Copenhagen, 2300, Denmark; 2Department of Infectious Diseases, Copenhagen University Hospital – Rigshospitalet, Copenhagen, 2100, Denmark; 3Departments of Epidemiology and Global Health, School of Public Health, Boston University, Boston, MA, USA; 4Department of Clinical Epidemiology, Aarhus University, Aarhus N, 8200, Denmark; 5Statens Serum Institut, Copenhagen, 2300, DenmarkCorrespondence: Niels Obel, Department of Infectious Disease Epidemiology and Prevention, Statens Serum Institut, Artillerivej 5, Copenhagen, 2300, Denmark, Tel +4532683252, Email niob@ssi.dkBackground: Observational studies of SARS-CoV-2 vaccine effectiveness are prone to confounding, which can be illustrated using negative control methods.Methods: Nationwide population-based cohort study including two cohorts of Danish residents 60– 90 years of age matched 1:1 on age and sex: A vaccinated and a non-vaccinated cohort, including 61052 SARS-CoV-2 vaccinated individuals between 1 March and 1 July 2021 and 61052 individuals not vaccinated preceding 1 July 2021. From these two cohorts, we constructed negative control cohorts of individuals diagnosed with SARS-CoV-2 infection or acute myocardial infarction, stroke, cancer, low energy fracture, or head-trauma. Outcomes were SARS-CoV-2 infection, negative control outcomes (eg, mammography, prostate biopsy, operation for cataract, malignant melanoma, examination of eye and ear), and death. We used Cox regression to calculate adjusted incidence and mortality rate ratios (aIRR and aMRR).Results: Risks of SARS-CoV2 infection and all negative control outcomes were elevated in the vaccinated population, ranging from an aIRR of 1.15 (95% CI: 1.09– 1.21) for eye examinations to 3.05 (95% CI: 2.24– 4.14) for malignant melanoma. Conversely, the ri