Epidemiology of Thrombotic Thrombocytopenia Syndrome in Integrated Health-care Database in England

Background/Rationale: A rare syndrome of thrombosis associated with low platelets has been reported in a few cases of recent exposure to COVID-19 vaccine. Medicines & Healthcare products Regulatory Agency (MHRA) had requested for all cases of thrombosis or thrombocytopenia occurring within 28 days of coronavirus vaccine to be reported via the online yellow card system. This syndrome seems to be affecting patients of all ages and both genders; at present there is no clear signal of risk factors. Up to and including 14 April 2021, the MHRA had received 168 United Kingdom (UK) reports of thrombo-embolic events with concurrent low levels of platelets following the use of the COVID-19 Vaccine AstraZeneca. The United Kingdom (UK) is uniquely placed to study this area because of its registration-based primary care system, and a unique identifier number links primary care to secondary care data. Additionally, vaccination is well advanced, maximising population wide vaccine exposure.

Objectives and Hypotheses: Estimate event rates and describe characteristics of patients with a record for thrombotic thrombocytopenia syndrome, thromboembolism or thrombocytopenia, in the general population of England.

Methods:

Study design: Secondary data analysis using a cohort design. Data Source(s): This is a retrospective cohort study using linked secondary databases in England accessed through the NHS Digital Trusted Research Environment (TRE). The primary care data will be linked with vaccination, primary care data, hospitalization, COVID-19 test results, mortality data at the national level for capture of key study variables. This integrated digital health system will also include other linkages such as the Oxford-Royal College of General Practitioners sentinel network; ORCHID.

Study Population: All patients, with our population of interest in England who are present in the integrated health records of NHS Digital TRE and/or the Oxford Royal College of General Practitioners Clinical Informatics Digital Hub (ORCHID) database at the start of each study period.

Variables: Demographic, socioeconomic, clinical descriptors and risk factors for thrombosis and/or thrombocytopenia; COVID-19 vaccines.

Statistical Analysis: The event rates with 95% CIs will be calculated by dividing the number of events with person-time at risk per 100,000 person-years. Further, event count and event rates will be evaluated in a relationship with COVID-19 vaccination date.