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 |
Most, but not all key confounders were taken account of in the analyses (Covid symptoms at time of potential vaccination were not taken account of) |
| 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. |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Low |
No concerns in this domain - data available for all participants |
| Measurement of the outcome |
Moderate |
Potential risk of bias arising from the testing being influenced by knowledge of vaccination status in patients admited to hospital and not being tested prior to hospitalization. |
| Selection of the reported results |
Low |
Statistical plan available and analaysis conducted in ccordance to pre-deteremine analysis plan |
| Overall risk of bias |
Moderate |
|
| Overall comment | In addition to the published report, the protocol and supplementary material were used in data extraction and risk of bias assessment. The average prevalence of variants was not reported; among variants of concern, Alpha comprised the largest subset during the study period across the regions sampled.
Some concerns over the possibility of uncontrolled confounding given the observational nature of the data and of outcome ascertainment being influenced by knowledge of vaccination status. |