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 |
The analysis adjusted for most, but not all, key confounders. COVID-19 symptoms at the time of potential vaccination were not taken account of. |
| 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. Additionally, sequencing failed for ~36% specimens. Evidence as to whether vaccination is associated with lower viral loads and thus a greater likelihood of sequencing failure is, however, inconclusive. |
| 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 |
Low |
No particular concerns in this domain - analyses of electronic medical records and data tables do not raise concerns with regard to missing data. |
| Measurement of the outcome |
Moderate |
Infection outcomes: Moderate risk of bias - Potential bias for infection outcomes due to differential testing according to vaccination status. Severe disease: Low risk of bias - No particular concerns for this outcome. |
| Selection of the reported results |
Moderate |
There is no evidence of a protocol, and we have concerns that results could have been selected for reporting because of the findings. |
| Overall risk of bias |
Moderate |
|
| Overall comment | Potential for residual confounding, selection bias (e.g. due to the test-negative design), concerns around potential differential testing (for infection outcomes only), and lack of a pre-determined analysis plan. |