Strengths and limitations
SEQUELAE
There are several strengths to our study. First, we emulated a hypothetic trial using a population-based electronic database to evaluate the comparative effectiveness of two different vaccines against COVID-19. Second, the findings are consistent with the indirect comparison of the effectiveness of these two vaccines based on the results of their respective clinical trials. Third, results from the sensitivity analyses using negative control outcomes confirmed that the effect of residual confounding, if present, is likely to be minimal, supporting the validity of our main findings. Fourth, considering that the time-varying confounders (e.g., COVID-19 vaccines where rollout in the period specified started in the oldest age groups) [However, our study also has some limitations. First, although we used rigorous approaches to control for confounding (i.e., demographic characteristics, medical history, comorbidities, and health care utilization), unmeasured residual confounding, such as the severity of comorbidities, cannot be ruled out. Second, owing to a lack of detailed information in IMRD, we are unable to assess the comparative effectiveness of two vaccines on several other sequelae of COVID-19, such as intensive care unit admission, mechanical ventilation, or length of hospitalisation. Future studies are needed to evaluate these additional outcomes. Third, we cannot access the data held in the hospital and were not reported back to GPs (e.g., tests were performed at the hospital and were not reported back to GPs). Thus, misclassification of the COVID-19 diagnosis could occur and bias the study findings. However, such bias, if occurred, is likely to be small and non-differential. As a result, it would bias the observed associations towards the null. Moreover, although the original PCR test results were not available and the sensitivity for capturing COVID-19 cases through the Read Code has not been validated in the IMRD, a recent study of COVID-19 codes recorded in the primary care setting suggested that clinical diagnosis of COVID-19 by physicians followed a similar trend to test positive cases confirmed by the UK national testing service [