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 |
The analysis adjusted for age, race and ethnic group, presence of at least one underlying condition or risk factor for severe COVID-19, and close contact with patients with COVID-19 in the workplace or with persons with COVID-19. The authors did not adjust for variables that resulted in a change of less than 10% in the model estimate for vaccination status (including sex and education which proxied for socioeconomic status). They also did not adjust for symptoms at the time of potential vaccination or hospitalization and need for health care. |
| 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 |
Low |
No particular concerns in this domain - vaccination status is likely to be adequately measured. Information on COVID-19 vaccination dates and products received was obtained from occupational health clinics, vaccine cards, state registries, or medical records. |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Low |
There were minimal missing data. |
| Measurement of the outcome |
Low |
No particular concerns in this domain - determination of infection is unlikely to be biased. |
| Selection of the reported results |
Low |
A protocol is available and consistent with the result presented. |
| Overall risk of bias |
Moderate |
|
| Overall comment | Some concerns over uncontrolled confounding; in addition there are uncertainties over the possibility of selection bias in a test-negative design. |