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, diabetes mellitus, renal, hepatic, neurologic, hematologic,cardiovascular and respiratory chronic diseases, obesity, immunosuppressed status and time (day of year). |
| 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. |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Moderate |
Ethnicity is missing for 18% of individuals. This is not discussed in the paper. |
| Measurement of the outcome |
Low |
No particular concerns in this domain. |
| Selection of the reported results |
Low |
There is a protocol available from a previous study, which the authors declare they have followed. |
| 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 and in relation to missing data. |