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, health-seeking behaviour (by design), comorbidities, calendar time, need for health care (disease requiring oxygen therapy), and also for occupational role. There remains a likelihood of uncontrolled confounding, for example by socio-economic status and ethnicity, and because some of the variables included may not have been measured accurately. |
| Selection of participants into the study |
Serious |
Participation rates were notably different between those testing positive and those testing negative, and this may have been influenced by vaccination status. In addition, 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 |
Serious |
Vaccination status was self-reported over the telephone after outcomes had occurred, introducing the possibility of recall (or reporting) bias. |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Low |
The results tables indicate there were no missing data. |
| 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 |
Serious |
|
| Overall comment | Concerns over uncontrolled confounding, self-report of vaccination status and differential participation rates between those who tested positive and those who tested negative. |