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, gender, ethnicity, socioeconomic status and vaccination manufacturer. Geographic location was similar due to restriction of the study design. Our prespecified important confounding domains of health-seeking behaviour, comorbidities and symptoms at time of vaccine were not controlled for, leading to a likelihood of uncontrolled confounding. |
| Selection of participants into the study |
Moderate |
Some patients were excluded from main analysis because they died (all causes), although no numbers are given. It is unclear what impact this might have, but it is likely minimal. Selection bias may also occur because the vaccinated group are entered into the intervention upon the date of their vaccine, whereas their previous time unvaccinated is not included in the analysis. |
| 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 |
Low |
No concerns (<1% of missing vaccine type; and 14% ethnicity data missing, but this wasn’t excluded, just considered as an unknown ethnicity stratum) |
| 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 around lack of controlling for important potential confounders, possible selection bias and excluding patients who died. |