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 |
| Randomization |
Low |
Quote: "Eligible participants were assigned to groups using stratified permuted block randomization with a 5:1 allocation ratio (250 to the vaccine booster group, 50 to the placebo group) by R-CRAN-version 4.0.1. There were three strata for randomization based on the primary vaccine platform: received. Adenoviral vector (including ChAdOx1 or Sputnik V), inactivated whole virus (BBIBP-CorV or COVIran Barekat) or recombinant protein (SpikoGen®). Most of the SpikoGen®-prime participants were past participants of the SpikoGen® Phase 2 clinical trial. The appearance of the vaccine and the placebo were identical, and the participants, investigators, and laboratory staff were masked to the treatment allocation."
Comment: Allocation sequence probably random. Allocation sequence concealed. Imbalances in baseline characteristics appear to be compatible with chance. |
| Deviations from intervention |
Low |
Quote: “The appearance of the vaccine and the placebo were identical, and the participants, investigators, and laboratory staff were masked to the treatment allocation.“
Comment: Blinded study (participants and personnel/carers). Safety outcomes were reported for all randomized participants. As we are assessing the effect of assignment to intervention, the analysis method performed on these safety outcomes, was considered appropriate. Risk assessed to be low for the outcomes: Local adverse events. Systemic adverse events. Adverse events. Serious adverse events. |
| Missing outcome data |
Low |
Comment: 300 participants randomized; 300 participants analyzed.
Data available for all or nearly all participants randomized. Risk assessed to be low for the outcomes: Local adverse events. Systemic adverse events. Adverse events. Serious adverse events. |
| Measurement of the outcome |
Low |
Comment: Method of measuring the outcome probably appropriate.
Measurement or ascertainment of outcome probably does not differ between groups. Blinded study (outcome assessor). Risk assessed to be low for the outcomes: Local adverse events. Systemic adverse events. Adverse events. Serious adverse events. |
| Selection of the reported results |
Low |
Comment: The prospective registry was available.
Outcomes pre-specified (Dated Dec 12th, 2021 and Jan 4th 2022) Results were not selected from multiple outcome measurements or analyses of the data. Trial analyzed as pre-specified. Risk assessed to be low for the outcomes: Local adverse events. Systemic adverse events. Adverse events. Serious adverse events. |
| Overall risk of bias |
Low |