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, comorbidities, contact setting (household/other), vaccination status of index case and calendar month. Uncontrolled confounding is likely, for example due to COVID-19 symptoms at the time of potential vaccination (although less serious for test-negaive study designs) and socioeconomic status. |
| 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. Participants (not cases) were excluded if they had a prior positive test, were nursing home residents or did not complete the testing protocol. Given these could also be associated with the outcome, this could be a concern for selection bias. |
| Clasification of interventions |
Low |
No particular concerns in this domain - vaccination status is likely to be adequately measured (through enhanced epidemiological surveillance of COVID-19). |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Low |
Data are likely to be fairly complete - information on vaccination status, outcomes and confounders is based on health records. |
| Measurement of the outcome |
Low |
No specific concerns. For infection outcomes, propensity to get tested is unlikely to depend on vaccination status as this is a test negative design. |
| Selection of the reported results |
Moderate |
There is no evidence of a protocol, 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 the lack of adjustment for important confounders, and no pre-defined statistical plan available. |