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 sex and epidemiological week. Geographic location and age were similar due to restriction of the study design. Our prespecified important confounding domains of socioeconomic status, health-seeking behaviour, symptoms, comorbidities and ethnicity were not controlled for, leading to a likelihood of uncontrolled confounding. |
| Selection of participants into the study |
Moderate |
Single vaccinated individuals were excluded from analysis, which is likely to introduce some bias. |
| Clasification of interventions |
Low |
No concerns in this domain - vaccination status is likely to be adequately measured (obtained from electronic health records). |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Low |
Data appear to be complete for vaccination status, confounders and outcomes. |
| Measurement of the outcome |
Moderate |
Vaccinated individuals may be more or less likely than unvaccinated individuals to seek a test, particularly when asymptomatic. |
| 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, exclusion of single-vaccinated individuals and absence of a pre-determined analysis plan. |