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 age, sex, ethnicity, health-seeking behaviour (previous healthcare encounters), comorbidity (any vs none), calendar time. Uncontrolled confounding remains possible. |
| 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 |
Low |
Data are missing for some confounders, but proportions of missing data are low. |
| 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 |
Low |
A protocol is available and consistent with the result presented. |
| Overall risk of bias |
Moderate |
|
| Overall comment | Some concerns over uncontrolled confounding; in addition there are uncertainties over the possibility of selection bias in a test-negative design. |