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, baseline date, home maker service, place of birth, comorbidities and education. Uncontrolled confounding is likely in this cohort study, for example due to COVID-19 symptoms at the time of potential vaccination, health-seeking behaviour, geographical location and ethnicity. |
| Selection of participants into the study |
Moderate |
Some concern in this domain due to exclusion of unvaccinated participants when receiving first dose, those previously infected and also those who died within 14 days. 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 (using national electronic health records). |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Low |
No concerns in this domain – no missing data, healthcare providers reporting 100% coverage. |
| Measurement of the outcome |
Moderate |
For the symptomatic outcome: propensity to get tested might depend on vaccination status and is therefore classed as a moderate risk of bias. However this is unlikely to be an issue for severe disease (low risk of bias). |
| Selection of the reported results |
Moderate |
There is no evidence of a protocol or statistical 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 over the lack of adjustment for important confounders, potential misclassificiation of the symptomatic outcome, and exclusion of those previously infected or who died within 14 days. |