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 was adjusted for age, sex, ethnicity, geographic location and calendar time, but not for socioeconomic status, specific populations, comorbidities, symptoms at the time of vaccination or hospitalization and healthcare needs. The authors stated that adjustment for these variables was not found to substantially influence the effect estimate, but results were not reported. |
| Selection of participants into the study |
* |
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 |
* |
An unknown proportion of participants had self or proxy reported vaccination status after the time of intervention, which may lead to bias due to misclassification of intervention. |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Moderate |
No information provided on missing data |
| Measurement of the outcome |
Low |
No particular concerns in this domain - determination of infection unlikely to be biased. |
| Selection of the reported results |
Low |
Analysis conducted in accordance to pre-specified analysis plan available in the protocol |
| Overall risk of bias |
Serious |
|
| Overall comment | Concerns about uncontrolled confounding; factors such as socio-economic status and comorbidities were not adjusted for. |