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 group, sex, epidemiological week, and health authority (HA) of residence. Uncontrolled confounding is likely, due to socio-economic status, co-morbidities, health-seeking behaviour (though not such a concern with a test negative design), specific populations, or covid symptoms at time of potential vaccination (though not such a concern with case-control studies). |
| Selection of participants into the study |
Serious |
Selection bias may have been introduced due to the approach of selecting individuals with complete data and exclusion of those “tested outside of public funding", those who received > 1 vaccine dose (mRNA) and non-clinical screening samples. The authors do not present data on the selection of cases and controls into the study or how many exclusions were made before arriving at the total of 16,993 SARS-CoV-2 individuals which contributed to the analysis. |
| Clasification of interventions |
Low |
No particular concerns in this domain - vaccination status is likely to be adequately measured. |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Low |
No concerns in this domain - the authors selected participants into the study in part based on the availability of data on intervention status, outcomes and potential confounders. |
| Measurement of the outcome |
Low |
No particular concerns in this domain – test-negative design with laboratory-confirmed outcome (PCR test). |
| Selection of the reported results |
Moderate |
There was no evidence of a protocol/analysis plan. |
| Overall risk of bias |
Serious |
|
| Overall comment | Concerns regarding uncontrolled confounding and bias in selection of participants into the study. |