Background rates of hospitalizations and emergency department visits for selected thromboembolic and coagulation disorders in Ontario, Canada, 2015 to 2020, to inform COVID-19 vaccine safety surveillance
Authors: Nasreen Sharifa , Calzavara Andrew , Sundaram Maria , MacDonald Shannon E. , Righolt Christiaan , Pai Menaka , Field Thalia , Zhou Lily W. , Wilson Sarah , Kwong Jeffrey C. 
Published: April 2021

AbstractObjectiveThe objective of this study was to estimate background rates of selected thromboembolic and coagulation disorders in Ontario, Canada.DesignPopulation-based retrospective observational study using linked health administrative databases. Records of hospitalizations and emergency department visits were searched to identify cases using diagnostic codes from the International Statistical Classification of Diseases and Related Health Problems, Tenth Revision, Canada (ICD-10-CA).ParticipantsAll Ontario residents.Primary outcome measuresIncidence rates of stroke, deep vein thrombosis, pulmonary embolism, idiopathic thrombocytopenia, disseminated intravascular coagulation, and cerebral venous thrombosis during five pre-pandemic years (2015–2019, annually, averaged, and monthly average) and 2020.ResultsThe average annual population was 14 million with 51% female. The mean annual rates during 2015–2019 were 127.1/100,000 population (95% confidence interval [CI], 126.2, 127.9) for ischemic stroke, 22.0/100,000 (95%CI, 21.6, 22.3) for intracerebral haemorrhage, 9.4 (95%CI, 9.2, 9.7) for subarachnoid haemorrhage, 86.8/100,000 (95%CI, 86.1, 87.5) for deep vein thrombosis, 63.7/100,000 (95%CI, 63.1, 64.3) for pulmonary embolism, 6.1/100,000 (95%CI, 5.9, 6.3) for idiopathic thrombocytopenia, 1.6/100,000 (95%CI, 1.5, 1.7) for disseminated intravascular coagulation, and 1.5/100,000 (95%CI, 1.4, 1.6) for cerebral venous thrombosis. Rates were lower in 2020 than during the pre-pandemic years for ischemic stroke, deep vein thrombosis, and idiopathic thrombocytopenia. Rates were generally consistent over time, except for pulmonary embolism, which increased from 57.1 to 68.5 per 100,000 between 2015 and 2019. Rates were higher for females than males for subarachnoid haemorrhage, pulmonary embolism, and cerebral venous thrombosis, and vice versa for ischemic stroke and intracerebral haemorrhage. Rates increased with age for most of these conditions, but idiopathic thrombocytopenia demonstrated a bimodal distribution with incidence peaks at 0–19 years and ≥60 years.ConclusionsOur estimated background rates help to contextualize observed events of these potential adverse events of special interest and to detect potential safety signals related to COVID-19 vaccines.Strengths and limitations of this study➢Recent background rates of selected thromboembolic and coagulation disorders that are potential adverse events special interest related to COVID-19 vaccine are estimated.➢Background rates during five pre-pandemic (2015–2019) years and 2020 will provide context for these events to identify vaccine safety signals.➢We used recorded diagnostic codes in administrative data without information on clinical and/or diagnostic confirmation, and the validity of these data are imperfect, which may result in under or overestimation.