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 matched and adjusted only for age and postcode. Our prespecified important confounding domains of comorbidity, geographic location, and COVID-19 symptoms at the time of potential vaccination were not controlled for, leading to a likelihood of uncontrolled confounding. |
| Selection of participants into the study |
Moderate |
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. |
| Clasification of interventions |
Moderate |
Information was recorded at the time of vaccination for everyone vaccinated in Oregon. However, anyone who was vaccinated outside of the state would not have their vaccination recorded in the state records and may be misclassified as unvaccinated. |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Low |
Data appear to be complete for vaccination status, measured confounders and outcomes. 72/1228 cases were excluded who had their +ve test from point-of-care information rather than electronic health records. |
| 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 uncontrolled confounding. |