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, and calendar week. Uncontrolled confounding is likely, for example due to comorbidities, covid symptoms at the time of potential vaccination (and previous COVID-19 infection) and socioeconomic status. |
| Selection of participants into the study |
Low |
No particular concerns in this domain. |
| 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 particular concerns in this domain - data from national electronic health records and seems complete. |
| Measurement of the outcome |
Serious |
Testing was not required for those who received two doses and therefore the infection after two doses outcome is at a serious risk of bias. For the remaining infection and symptomatic outcomes: 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 and mortality (low risk of bias). |
| Selection of the reported results |
Moderate |
Some concerns in this domain – protocol not available to assess pre-specified plan. |
| Overall risk of bias |
Serious |
|
| Overall comment | Covid related mortality extracted for all cause mortality.Concerns over the lack of adjustment for important confounders, potential misclassificiation of the infection and symptomatic outcomes, and no pre-defined statistical plan available. |