Note: The risk of bias by domain corresponds to the highest risk of bias among outcomes by domain.
The overall risk of bias corresponds to the overall highest risk of bias assessed among outcomes.
| Bias | Author's judgement | Support for judgement |
| Confounding |
Serious |
The analysis adjusted for age, sex, socioeconomic status (index of deprevation), number of prior COVID-19 tests and calendar time. Uncontrolled confounding is likely, for example due to cormorbidities, ethnicity and COVID-19 symptoms when a vaccination was due to be received. |
| Selection of participants into the study |
Moderate |
The study used a test-negative design. This has the potential to suffer from selection bias by being restricted to individuals getting a test, although the issues are not yet well understood. |
| Clasification of interventions |
Low |
No particular concerns in this domain - vaccination status is likely to be adequately measured. |
| Deviations from intervention |
Low |
No concerns in this domain - the study was obervational |
| Missing outcome data |
Low |
The database studied contains about 99% of the Scottish population, and we regard it likely that data were nearly complete for the variables analysed. |
| Measurement of the outcome |
Low |
No particular concerns in this domain - determination of infection unlikely to be biased. |
| Selection of the reported results |
Moderate |
There is no evidence of an analysis plan, and we have concerns that results could have been selected for reporting because of the findings. |
| Overall risk of bias |
Serious |
|
| Overall comment | Concerns mainly over uncontrolled confounding; in addition there are uncertainties over the possibility of selection bias in a test-negative design. |