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 |
Moderate |
Most key confounders adjusted for. However, no adjustment was made for COVID-19 symptoms at time of potential vaccination. |
| 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. Also, there were some issues with inconsistent recording of symptom data - only those with symptoms recorded were eligible for inclusion. |
| 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 observational. |
| Missing outcome data |
Moderate |
Data on vaccination status appear complete and data on confounders were missing for <1% of participants. However, information on variant type was missing for 20% of participants. |
| Measurement of the outcome |
Moderate |
No particular concerns in this domain for determination of disease status but some potential misclassification of severe disease outcomes due to unlinked case records and incomplete capture of severe outcomes in the databases analysed. |
| Selection of the reported results |
Moderate |
There is no protocol available. |
| Overall risk of bias |
Moderate |
|
| Overall comment | Some concerns regarding residual confounding, bias in the selection of participants into the study, missing data on variant type and assessment of the severe disease outcome. |