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, gender, calendar week and history of past SARS-CoV-2 infection. Geographic location, health-seeking behaviour and specific subpopulation (e.g. healthcare worker/elderly in institution/pregnancy) were similar due to restriction of the study design. Our pre-specified important confounding domains socioeconomic status, ethnicity, comorbidities, symptoms at time of planned vaccination and hospitalization and need for health care were not controlled for, leading to a likelihood of uncontrolled confounding. |
| Selection of participants into the study |
Low |
No particular concerns in this domain – study participants consisted in all residents of long-term care facilities in which any SARS-CoV-2 outbreak that implicated B.1.351 had been documented. |
| 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 particular concerns in this domain – just 2 out of 376 participants are missing from the analysis. |
| Measurement of the outcome |
Low |
No particular concerns in this domain. Although outcome assessors likely knew the patients’ vaccination status, this is unlikely to bias the determination of infection and disease severity as study participants were long term care residents and testing was part of a surveillance protocol. |
| Selection of the reported results |
Moderate |
There is no evidence of a protocol, analysis plan or study registry and we have concerns that results could have been selected for reporting because of the findings. |
| Overall risk of bias |
Serious |
|
| Overall comment | Concerns mainly about the possibility of uncontrolled confounding. |