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 controlled for age (collapsed into a categorical variable) and calendar time. Many important baseline confounders were not adjusted for, including sex, socioeconomic status, comorbidities, symptoms, health seeking. |
| Selection of participants into the study |
Low |
No concerns in this domain. |
| Clasification of interventions |
Low |
Sensitivity analysis was done assuming misclassification of 7.3% of vaccinated cases as unvaccinated. This reduced the vaccine effectiveness estimates by 1% to 2%. However, main results included only known data, so we have low concerns of bias. |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Low |
No concerns in this domain: it is likely data are fairly complete given that information on vaccination status, outcomes (and confounders) is based on health records. |
| Measurement of the outcome |
Low |
No concerns in this domain. |
| Selection of the reported results |
Low |
There is no evidence of a protocol/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 the lack of adjustment for important confounders, and no pre-defined statistical plan is available. |