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 round of the study (REACT), age, sex, index of multiple deprivation (an area level measure of socioeconomic status), region and ethnicity. It did not control for specific populations, comorbidities, COVID-19 symptoms at the time of potential vaccination or hospitalisation 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. |
| Deviations from intervention |
Low |
No concerns in this domain - the study is observational. |
| Missing outcome data |
Low |
No concerns in this domains – data appear to be complete for vaccination status, confounders and outcomes. |
| Measurement of the outcome |
Low |
No particular concerns in this domain - determination of infection unlikely to be biased. |
| Selection of the reported results |
Low |
No concerns in this domain - the analysis is conducted in accordance with a pre-specified plan. |
| Overall risk of bias |
Serious |
|
| Overall comment | The pre-print article was used in data extraction and risk of bias assessment. A protocol is available. This is a cross-sectional study of random samples in different rounds of self-administered testing in England. Here VE adjusted for round, age, sex, IMD, region, and ethnicity for rounds 13 and 14 (24 June to 12 July and 9 to 27 September 2021) were reported.
Concerns about uncontrolled confounding; factors such as specific populations, comorbidities, COVID-19 symptoms at the time of potential vaccination, and need for health care were not adjusted for. |