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, sec, ethnicity, geographic behaviour and calendar time. Comorbidities differed between vaccinated and unvaccinated participants, but these do not seem to have been adjusted for. |
| 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 observational. |
| Missing outcome data |
Moderate |
There is no information on missing data. |
| Measurement of the outcome |
Low |
No particular concerns in this domain - determination of the outcomes are unlikely to be biased. |
| 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 about uncontrolled confounding (comorbidities were unbalanced between vaccination groups); in addition, there are uncertainties over the possibility of selection bias in a test-negative design and in relation to the missing data. |