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 |
The analysis adjusted for age, sex, time since receipt of the second dose, comorbidities (diabetes, immunosuppression status, BMI≥30 , CVD) and number of positive tests performed in the population. Uncontrolled confounding is likely, for example arising from ethnicity, geographic location, covid symptoms at the time of vaccination and hospitalisation and need for healthcare. |
| Selection of participants into the study |
Moderate |
The study used a test-negative design. This has the potential to suffer from selection bias by being restricted to individuals getting a test, although the issues are not yet well understood. |
| Clasification of interventions |
Low |
No concerns identified in this domain. |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Low |
No information is provided on missing data. MHS EMR data are likely to be fairly complete for the variables that were controlled and probably almost entirely complete for vaccination status and dates. |
| 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 is no evidence of an analysis plan, and we have concerns that results could have been selected for reporting because of the findings. |
| Overall risk of bias |
Moderate |
|
| Overall comment | The preprint was used in data extraction and risk of bias assessment. Results were extracted for the longest follow up time, which was 14-20 days after booster. Prevalence of Delta variant was not reported, but states it was the dominant strain in Israel.
Concerns over uncontrolled confounding. We have concerns due to the use of a test-negative design and a lack of a statistical analysis plan. |