In order to help public health authorities and clinicians to contextualize observed events of vaccine-induced immune thrombotic thrombocytopenia (VITT) – an extremely rare but serious blood clot following immunization with the AstraZeneca/Vaxzevria COVID-19 vaccine – the Canadian Immunization Research Network (CIRN), including CITF-funded researcher Dr. Jeff Kwong, sought to estimate background rates of selected thromboembolic and coagulation disorders in Ontario from 2015-2020. Without the amalgamation of this data, comparison between existing thromboembolic and coagulation disorder rates and VITT are difficult. This analysis is intended to help health care professionals to assess potential vaccine safety signals. It first appeared as a preprint, and is now published in BMJ Open.

Records of hospitalizations and emergency department visits in Ontario were searched to identify cases. Primary outcome measures used were incidence rates of ischemic stroke, intracerebral haemorrhage, subarachnoid haemorrhage, deep vein thrombosis, pulmonary embolism, idiopathic thrombocytopenia, disseminated intravascular coagulation, and cerebral venous thrombosis.

Key points

  • The rates of thromboembolic and coagulation disorders were relatively stable during the pre-pandemic period but were lower in 2020 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. However, this could be due to changes in changes in clinical or coding practices over time rather than a true increase of the disease.
  • Event rates increased with age for most of these conditions, but idiopathic thrombocytopenia demonstrated a bimodal distribution with incidence peaks at 0–19 years and over 60 years.

Using pre-COVID-19 rates of thromboembolic and coagulation disorders, public health authorities and clinicians can calculate expected numbers of events. They will be in a better position to contextualize adverse clotting events associated with COVID-19 vaccines by comparing observed and expected numbers of events to identify potential safety signals. The data could also be a useful tool to maintain vaccine confidence when discussing vaccine hesitancy with their patients.

Nasreen S, Calzavara AJ, Sundaram ME, MacDonald, SE, Righolt CH, Pai M, Field TS, Zhou LW, Wilson SE, Kwong JC, Background incidence rates of hospitalisations and emergency department visits for thromboembolic and coagulation disorders in Ontario, Canada for COVID-19 vaccine safety assessment: a population-based retrospective observational study. BMJ Open 2021; 11:e052019. doi: 10.1136/bmjopen-2021-052019