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, sex and risk of exposure to covid-19 positive individuals. There remains a likelihood of uncontrolled confounding, for example by socio-economic status, ethnicity, commorbidities. |
| Selection of participants into the study |
Moderate |
Test-negative design |
| Clasification of interventions |
Serious |
Vaccination status was collected by telephone interview after outcome assessment, and recall or reporting may have been affected by the outcome. |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Moderate |
There was substantial missing data due to people declining telephone interviews but small difference in response rate between test positive and test negative |
| 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 about uncontrolled confounding and potential for misclassification of vaccination status (collected by telephone interview after outcome assessment). |