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 and day of the week. Vaccine effectiveness was reported for a given day. The authors did not account for socioeconomic status, ethnicity, geographic location, health-seeking behaviours, specific populations, comorbidities, covid symptoms at the time of potential vaccination or hospitalization and need for health care. |
| Selection of participants into the study |
Low |
No particular concerns in this domain. |
| Clasification of interventions |
Low |
No concerns in this domain - vaccination status is likely to be adequately measured (obtained from electronic health records). |
| Deviations from intervention |
Low |
No concerns in this domain - the study is observational. |
| Missing outcome data |
Low |
No concerns in this domain. |
| Measurement of the outcome |
Moderate |
There was potential for differential measurement error in the outcome, since vaccinated individuals may be differentially likely to seek a test, particularly when asymptomatic. |
| Selection of the reported results |
Moderate |
There is no evidence of a protocol or statistical 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 uncontrolled confounding and possibility of differential misclassification of the outcome. |