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 was adjusted for age, sex, role, working context and week, but not not for ethnicity, socioeconomic status (although job role is a partial proxy), co-morbidities or symptoms at the time of potential vaccination. Further, there was an observed vaccine effectiveness during days 0-14 after dose 1 where none is expected, suggesting uncontrolled confounding. |
| Selection of participants into the study |
Moderate |
Selection restricted to those with symptoms. We cannot rule out the possibility of selection bias. |
| 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 |
No concerns in this domain - the percentage of missing data was low. |
| Measurement of the outcome |
Moderate |
Symptomatic infection: Test-seeking behaviour might vary according to vaccination status, which means a case may be more likely to be detected when assessing vaccine effectiveness on symptomatic infection. Infection: no particular concerns in this domain. |
| 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 about uncontrolled confounding; factors such as socio-economic status and comorbidities were not adjusted for. |