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 study matched individuals by age, sex, race, ethnicity, state of residence, health-seeking behaviors (SARS-CoV-2 PCR testing history), time (Date of vaccination). Uncontrolled confounding remains likely, for example by socio-economic status and comorbidities. |
| Selection of participants into the study |
Serious |
The study says it is restricted to individuals who received a PCR test. Information is not provided about why individuals received the PCR test. Receipt of the test could be influenced by vaccination status and by acquisition of COVID-19, introducing selection bias. |
| Clasification of interventions |
Low |
No particular concerns in this domain - vaccination status is likely to be adequately measured; analyses were restricted to individuals from Minnesota due to differences in linkage of vaccination registry data for the Mayo Clinic sample. |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Moderate |
Information is not provided about availability of data for analyses (particularly confounders). |
| Measurement of the outcome |
Moderate |
For severe disease (requiring hospitalization), it is possible (though probably not likely) that the decision to admit a patient was affected by knowledge of vaccination status. For other outcomes, any misclassification should not be related to vaccination status. |
| 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 | There is likely to be uncontrolled confounding, and we have a concern about selection bias because the analysis is restricted to individuals who had a PCR test, and this could be influenced by both vaccination and infection (sometimes referred to as collider bias). |