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: "An interactive Web-based response system was used for randomization, and the randomization lists were generated by an independent statistician using SAS (version 9.4)"
Comment: Allocation sequence random. Allocation sequence concealed. Risk assessed to be low. |
| Deviations from intervention |
Some concerns |
Quote "We masked participants, investigators, laboratory staff, and outcome assessors to the allocation of treatment groups ... Designated unblinded personnel were responsible for the preparation and administration of the vaccination, and were forbidden to reveal the identity of the study vaccines to the participants or other investigators"
Comment: Blinded study (participants and personnel/carers). Per-protocol analysis was performed on the outcomes. Reasons for exclusion: three participants randomized to the Convidecia boost group crossed over to other groups. Two participants were wrongly administrated with a homogeneous boost dose of CoronaVac and were re-classified into the CoronaVac boost group. One participant had in fact only received one primary dose and was re-classified into the CoronaVac/Convidecia 2 dose group (additional arm in the study extracted separately). As we are assessing the effect of assignment to intervention (intention-to-treat effect), we considered that the data were analyzed inappropriately. There was probably no substantial impact of failure to analyze participants according to their randomized assignment due to the small number of participants that crossed over. Risk assessed to be some concerns for the outcomes: Specific antibody GMT. Neutralizing antibody GMT. Cellular immune response. Local adverse events. Systemic adverse events. Adverse events. Serious adverse events. |
| Missing outcome data |
Low |
Comment: 202 participants randomized; 198 participants analyzed for safety, specific antibody GMT and neutralizing antibody GMT/43 analyzed for cellular immune response
Data available for nearly all participants randomized for safety, specific antibody GMT and neutralizing antibody GMT Risk assessed to be low for the outcomes: Specific antibody GMT. Neutralizing antibody GMT. Local adverse events. Systemic adverse events. Adverse events. Serious adverse events. Data not available for all or nearly all participants randomized for cellular immune response No evidence that the result is not biased. Reasons: "PBMCs from blood samples of the first 50 participants before and at 14 d after the booster were used to evaluate cellular immunity." Missingness could not depend on the true value of the outcome. Risk assessed to be some concerns for the outcome: cellular immune response |
| 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: Specific antibody GMT. Neutralizing antibody GMT. Celullar immune response. Local adverse events. Systemic adverse events. Adverse events. Serious adverse events. |
| Selection of the reported results |
Low |
Comment: The protocol, statistical plan and trial registry were available (dated 19 May 2021).
Outcome pre-specified. 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: Specific antibody GMT. Neutralizing antibody GMT. Cellular immune response. Local adverse events. Systemic adverse events. Adverse events. Serious adverse events. |
| Overall risk of bias |
Some concerns |