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 sex, age, reason for PCR testing, nationality and calendar week. Uncontrolled confounding is likely, for example arising from socio-economic status, co-morbidities, health-seeking behaviour, specific populations, or covid 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. |
| Clasification of interventions |
Low |
No particular concerns in this domain - vaccination status is likely to be adequately measured (obtained from electronic health records). |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Low |
No particular concerns in this domain – data were reasonably complete. |
| Measurement of the outcome |
Low |
No particular concerns in this domain – test-negative design with laboratory-confirmed outcome (PCR test). |
| Selection of the reported results |
Moderate |
There is no evidence of a protocol/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 regarding residual confounding and bias in selection of participants into the study (test-negative design). |