Problem
You are interested in discovering the rate of thromboembolism with COVID-19 vaccines. The outcome is exceedingly rare, but it is very important from a safety perspective. You don't have the money to conduct an RCT, but your colleague will give you all of her old data, which consists of 5 RCTs-worth of patient data from patients who have received the COVID-19 vaccine or placebo. Some patients have developed thromboembolism, many have not.
Design an observational study to assess whether COVID-19 vaccines are associated with an increased risk of developing thromboembolism compared to placebo. Your answer should include your design (case control or cohort) as well as the PICO framework for your suggested study.