Inclusion Criteria:  Adults aged ≥21yo. Asymptomatic 24h before the administration of booster dose. Has no active or previous RT-PCR lab-confirmed COVID-19 diagnosis. Completed three months to six months after the second dose of BBIBP-CorV. Have at least one Antibody test done before receiving the BBIBP-CorV booster dose OR can be done if the participant is yet to receive the BNT162b2 booster dose. Tested negative using Rapid Antigen Detection Test on the day of receiving the booster (positive results will confirm with RT-PCR). Study participants must have the ability to give informed consent.  Exclusion Criteria:  Children aged <21yo. Symptomatic within 24h before the administration of booster dose. Has active or previous RT-PCR lab-confirmed COVID-19 diagnosis. Did not complete three months to six months after the second dose of BBIBP-CorV. Does not have at least one Antibody test done before receiving the BBIBP-CorV booster dose Tested positive using Rapid Antigen Detection Test on the day of receiving the booster (positive results will be confirmed with PCR). Patients unable to give informed consent.