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 |
Moderate |
Individuals were matched on age, sex, place of residence, number of chronic conditions considered risk factors for COVID-19, month, number of PCR tests performed in the previous 9 months. To be eligible, individuals had to have no contact with the health-care system in the three days before recruitment date. |
| Selection of participants into the study |
Low |
No concerns in this domain. |
| Clasification of interventions |
Low |
No concerns in this domain. |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Moderate |
A high proportion of matched pairs were censored when the control received their third dose. These controls are likely to be different from those who did not receive a third dose. |
| Measurement of the outcome |
Moderate |
No concerns regarding determination of severe disease or mortality. However, there is the potential for differential misclassification of infection status. |
| Selection of the reported results |
Low |
Analyses reported align with the published protocol. |
| Overall risk of bias |
Moderate |
|
| Overall comment | Main concerns are the potential for uncontrolled confounding (as the study is observational), the likelihood of informative censoring, and possible differential ascertainment of infection status. |