Response to: letter to “Self-controlled case series design in vaccine safety: a systematic review”

KEYWORDS:

• Self-controlled case series design
• a matched cohort study
• relative risk
• absolute risk
• reporting quality

Thank you for the constructive comments [1]. As mentioned, the estimate obtained from self-controlled case series (SCCS) is incidence rate ratio (IRR) or relative incidence (RI), which is the ratio of the incidence rate of events in the exposed period to the incidence rate of events in the unexposed period [2]. Since the IRR or RI is obtained from self-comparison, and the source population of the cases in the SCCS is unclear, it is unable to estimate the absolute risk from IRR or RI. However, 88 (84%) of the 105 SCCS studies we included adopted the IRR or RI estimate, and 12% (13/105) translated relative risk into absolute risk [3].

To obtain the measure of absolute risk, it is still recommended to add a matched cohort study. For example, in the study by Ioannis Katsoularis et al. which evaluated the association of COVID-19 with acute myocardial infarction and ischemic stroke [4], 86,742 COVID-19 cases were included in the SCCS study, and these cases combined with 348,841 controls matched by age, sex, and county of residence were used for the cohort study. In our review, 15 included SCCS studies were combined with cohort studies, but many studies did not fully utilize this advantage to calculate the absolute risk. It is recommended that in vaccine safety research, the cohort studies are used to calculate absolute risk, and SCCS studies are used to estimate the relative risk for better reducing bias from time-fixed confounders.

In addition, since there is no specific checklist of reporting quality for SCCS design, the item that concerns whether translate estimates of relative risk into absolute risk for a meaningful period was reported in RECORD-PE is not entirely appropriate for the SCCS design [5]. As mentioned in our article, we expect that the evaluation tools of bias and reporting quality which are suitable for SCCS design will soon be developed in the future [5].