Randomisation and masking
ADVERSE REACTIONS
The field investigation team (research assistants and study nurses) performed final participant eligibility screening before enrolling participants during the vaccination visit. The intervention (ie, third-dose vaccination with CoronaVac or BNT162b2) was randomly preassigned to each individual who passed initial eligibility assessment by the online questionnaire system REDCap. The randomisation was based on computer-generated sequences of random numbers produced by NHLL using R software under a block randomisation structure, with block sizes of 2, 4, and 6. Two separate random sequences were generated, one for participants who had previously received two doses of CoronaVac and another for BNT162b2; ie, the four study groups included participants who previously received two doses of CoronaVac and were randomly assigned to receive a third dose of CoronaVac (hereafter CC-C) or BNT162b2 (hereafter CC-B), and participants who previously received two doses of BNT162b2 and were randomly assigned to receive a third dose of CoronaVac (hereafter BB-C) or BNT162b2 (hereafter BB-B). For logistical reasons, individuals were randomly assigned to the intervention at the time vaccination appointments were made; ie, eligible individuals were provided a list of vaccination appointment dates for the same assigned intervention to choose from. There were dropouts after the vaccination appointment was made and before vaccination. However, both the participants and front-line study staff liaising with participants were not aware of the assigned intervention until after vaccination (ie, allocation was concealed) because the vaccination was administered by separate nurses for whom allocation was not concealed: the appointment dates were randomised by computer, and the study principal investigator informed the nurses of the assigned intervention for a vaccination appointment date only a few days beforehand. Nurses with knowledge of group allocation had no further involvement in the trial after administering the intervention. Participants and front-line study staff were unmasked to the type of vaccine received after vaccination through the mandatory government vaccination record. The staff conducting laboratory tests for immunogenicity data were masked to the third-dose allocation until after the laboratory tests were completed. Data on adverse reactions were self-reported by participants with knowledge of group allocation. Data on COVID-19 status based on self-administered rapid antigen tests were self-reported by participants with knowledge of group allocation.