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, age, region, employment type, ethnicity, household size and neighbourhood deprivation. Uncontrolled confounding is likely, for example arising from comorbidities and health-seeking behaviour. |
| Selection of participants into the study |
Low |
No particular concerns in this domain. |
| Clasification of interventions |
Serious |
Vaccination status is self-reported, so may suffer from recall bias. |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Moderate |
A large number of people did not know their vaccination status, possibly biasing the results. |
| Measurement of the outcome |
Low |
No particular concerns in this domain - determination of the outcome is 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 around failure to adjust for several important confounders and use of self-reported vaccination status, which had a large amount of missing data. |