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 study adjusted for age and calendar time by analysis and to a large extent for health-seeking behaviours, population sector and geographical location by design (study restricted to healthcare workers in a British Columbia district). Uncontrolled confounding remains likely, for example due to COVID-19 symptoms at the time of potential vaccination, hospitalization, comorbidities and socio-economic status. |
| Selection of participants into the study |
Low |
No concerns on this domain - participants selected into the study were all healthcare workers. |
| Clasification of interventions |
Low |
No concerns on this domain - vaccination status is likely to be assessed reliably. |
| Deviations from intervention |
Low |
No concerns in this domain - the study is observational. |
| Missing outcome data |
Low |
No concerns in this domain - no evidence of missing data. |
| Measurement of the outcome |
Low |
No particular concerns in this domain - determination of infection and disease unlikely to be biased. |
| Selection of the reported results |
Moderate |
There is no evidence of a protocol/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 | The published report was used in data extraction and risk of bias assessment. 93% received BNT162b2 and 7% mRNA-1273 vaccine. Prevalence of variants during study period was estimated from the outbreak.info website. |