The primary endpoint comprises a composite endpoint of either an increase in anti-RBD antibody titre to ≥500 IU/mL at day 14 post 4th dose booster vaccine in those with anti-RBD antibody titre of ≤500 IU/mL immediately before 4th dose booster vaccination or a 2-fold increase in anti-RBD antibody titre at day 14 following 4th dose vaccination in those with anti-RBD titre of ≥500 IU/mL immediately before 4th dose booster vaccination, as measured by quantitative immunoassay targeting the anti-RBD antibody.

The primary endpoint comprises a composite endpoint of either an increase in anti-RBD antibody titre to ≥500 IU/mL at day 14 post 4th dose booster vaccine in those with anti-RBD antibody titre of ≤500 IU/mL immediately before 4th dose booster vaccination or a 2-fold increase in anti-RBD antibody titre at day 14 following 4th dose vaccination in those with anti-RBD titre of ≥500 IU/mL immediately before 4th dose booster vaccination, as measured by quantitative immunoassay targeting the anti-RBD antibody.

The primary endpoint comprises a composite endpoint of either an increase in anti-RBD antibody titre to ≥500 IU/mL at day 14 post 3rd dose booster vaccine in those with anti-RBD antibody titre of ≤500 IU/mL immediately before 3rd dose booster vaccination or a 2-fold increase in anti-RBD antibody titre at day 14 following 3rd dose vaccination in those with anti-RBD titre of ≥500 IU/mL immediately before 3rd dose booster vaccination, as measured by quantitative immunoassay targeting the anti-RBD antibody.

The primary endpoint comprises a composite endpoint of either an increase in anti-RBD antibody titre to ≥500 IU/mL at day 14 post 3rd dose booster vaccine in those with anti-RBD antibody titre of ≤500 IU/mL immediately before 3rd dose booster vaccination or a 2-fold increase in anti-RBD antibody titre at day 14 following 3rd dose vaccination in those with anti-RBD titre of ≥500 IU/mL immediately before 3rd dose booster vaccination, as measured by quantitative immunoassay targeting the anti-RBD antibody.