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 only for calendar time (intensity of exposure measured as daily 7-day moving average of national detected cases). Uncontrolled confounding is likely (for example due to age, socio-economic status, symptoms at the time of planned vaccination). Participants were all health care workers and vaccination was not prioritized for specific groups, so we do not judge the lack of adjustment as leading to critical risk of bias. |
| Selection of participants into the study |
Low |
No particular concerns in this domain. |
| Clasification of interventions |
Low |
No particular concerns in this domain - there is no information about how intervention status data were collected but we judge this likely to be adequately measured (probably obtained from electronic health records in the Hospital centre). |
| Deviations from intervention |
Low |
No particular concerns in this domain. The study was observational. |
| Missing outcome data |
Low |
No particular concerns in this domain. Data were reasonably complete. |
| Measurement of the outcome |
Low |
No particular concerns in ths domain - PCR test is performed at days one and ten after exposure to case index. |
| 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 | * |