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 adjusted for age, sex, ethnicity, NHS region, socio-economic status (index of multiple deprivation), care home residence and week of symptom onset. Uncontrolled confounding is likely, arising from comorbidities, smoking and health-seeking behaviour. |
| 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 available for 90% of participants, with missingness not likely to be strongly related to outcome. |
| Measurement of the outcome |
Low |
No particular concerns in this domain - determination of infection is 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 |
Serious |
|
| Overall comment | Concerns mainly over uncontrolled confounding; in addition there are uncertainties over the possibility of selection bias in a test-negative design. |