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 |
Concerns regarding the lack of adjustment for important confounders |
| Selection of participants into the study |
Moderate |
Some concern due to potential exclusion of newly vaccinated people |
| Clasification of interventions |
Moderate |
Some concern due to time-changing vaccination status |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Moderate |
Some concern due to substancial data missing (only 43% of weekly data reports returned) |
| Measurement of the outcome |
Low |
No particular concerns regarding determination of infection |
| Selection of the reported results |
Moderate |
Some concern due to no protocol/statistical plan available to verify if statistic was conducted according to pre-defined plan |
| Overall risk of bias |
Serious |
|
| Overall comment | Concerns over the lack of adjustment for important confounders, time-changing vaccination status, potential exclusion of newly vaccinated people, missing data and not pre-defined statistical plan available. |