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, sex, and calendar week. Uncontrolled confounding is likely, for example due to comorbidities, covid symptoms at the time of potential vaccination and socioeconomic status. |
| Selection of participants into the study |
Moderate |
Cases were excluded from the analysis if they had received only one dose, or had received two doses and fewer than 7 days had passed since the second dose. Given these could also be associated with the outcome this could be a concern for selection bias. |
| Clasification of interventions |
Low |
No particular concerns in this domain - vaccination status is likely to be adequately measured (using electronic health records). |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Serious |
Infection and severe disease: Moderate risk of bias; Symptomatic disease: Serious risk of bias. There is insufficient information to assess the amount of missing data or the implications for risk of bias. in addition, a number of individuals had information missing on symptoms which was not addressed. |
| Measurement of the outcome |
Serious |
Infection and symptomatic disease: Serious risk of bias, since propensity to get tested might depend on vaccination status. Severe disease: Low risk of bias. |
| 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 |
Serious |
|
| Overall comment | Cases from 20 June to 17 July 2021 extracted from a PDF of a presentation from the Israeli Ministry of Health with methods from Haas EJ, et al. Lancet. 2021. 397(10287):1819-1829. Prevalence of Delta variant was not reported but states, "Follow-up period shifted to weeks when delta variant dominated sequenced strains in Israel". Concerns over the lack of adjustment for important confounders, potential exclusion of those who only received one dose or newly vaccinated people, and no pre-defined statistical plan available. |