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 most of the pre-defined confounder; however, confounders were self-reported via questionnaire with potential for recollection bias. |
| Selection of participants into the study |
Serious |
Very low participation rate among both cases and controls. Demographic differences between cases and controls. |
| Clasification of interventions |
Serious |
Intervention classification could have been missclassified as vaccination (i.e. status , type of vaccine and dates) was self-reported. |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Low |
No concerns in this domain -the amount of missing data on the intervention and confounders was low for both groups. |
| Measurement of the outcome |
Serious |
Among controls there could be participants who have had symptoms but did not seek for test; only people who come forward for testing (and test positive) will be classified as cases. |
| 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 self-reported intervention and confounding, selection of case and controls into the study and ascertainement of the outcome. |