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 study matched individuals by age, sex, ethnicity (nationality), calendar time (of PCR test) and reason for PCR testing. The test-negative design should account to some extent for health-seeking behaviour. For the outcomes of infection and severe disease, the analysis adjusted for the matching variables. Matching and adjusting for the reason for the PCR test may be problematic, since it may be affected by vaccination status. Uncontrolled confounding is likely, for example due to socioeconomic status and comorbitities. |
| 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 (from a Qatar nationwide digital health information platform). |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Low |
The database studied was a national one and it is likely that data were nearly complete for the variables analysed. |
| Measurement of the outcome |
Low |
No particular concerns in this domain - determination of infection unlikely to be biased, and influences of knowledge of vaccination on assessment of other outcomes is likely to be minimal. |
| Selection of the reported results |
Moderate |
There is no evidence of a protocol, or and analysis plan or and study registry and we have concerns that results could have been selected for reporting because of the findings. |
| Overall risk of bias |
Serious |
|
| Overall comment | Concerns mainly over uncontrolled confounding; in addition there are uncertainties over the possibility of selection bias in a test-negative design. |