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 |
Moderate |
Analysis adjusted for age, sex, public health unit region, period of test, number of recent SARS-CoV-2 tests, comorbidity, receipt of recent influenza vaccinations, Census dissemination area, non-health essential worker, persons per dwelling, and proportion of self-identified visible minorities. Uncontrolled confounding remains possible (e.g. due to BMI or presence of symptoms when vaccination is due to happen). |
| Selection of participants into the study |
Serious |
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. For the outcome of severe disease, cases were test-positive people with hospitalization or death and controls were symptomatic people who tested negative, so those who tested positive but did not have hospitalization or death are excluded from the analysis. |
| 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 obervational |
| Missing outcome data |
Low |
There was minimal 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 | Control for confounding was good but uncontrolled confounding cannot be ruled out; in addition there is a possibility of selection bias (particularly for the outcome severe disease), due to uncertainties over the validity of a test-negative design in this regard. |