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 apparently adjusted for age, sex, occupational category and number of doses. These may be sufficient for a comparison of two vaccines, but it is possible that there is important confounding if the two vaccines were administered at different times or to people with different prognoses in other respects, and there is insufficient information to judge this. |
| Selection of participants into the study |
Low |
No concerns identified in this domain. |
| 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 |
Approximately 10% of participants had missing data, and information was not provided about these. |
| 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 mainly about the possibility of uncontrolled confounding; there are also some missing data with a lack of further information about these. |