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, socioeconomic status (IMD quintiles), ethnicity, geographic location, health-seeking behaviour (by design), specific populations (care home residents, healthcare workers), comorbidities (being in a vulnerable group) and calendar time, and additionally for recent travel. This covers almost all of our prespecified confounding domains. |
| 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 |
15% of participants had missing data on ethnicity, and it is unclear these missing data were addressed. In addition, it is reported that 7% of participants did not have their records linked. |
| Measurement of the outcome |
Low |
No particular concerns in this domain - determination of infection unlikely to be biased. |
| Selection of the reported results |
Moderate |
There is no evidence of an analysis plan, and we have concerns that results could have been selected for reporting because of the findings. |
| Overall risk of bias |
Moderate |
|
| Overall comment | Some concerns over the possibility of uncontrolled confounding given the observational nature of the data, and uncertainty in how missing data were addressed. |