Signed informed written consent Able to follow the vaccination schedules, visits and tests General health and controlled underlying diseases (based on the physician's recommendation) Iranian citizenship Resident in Tehran Both sexes (male and female) Aged above 18 years Receiving 2 doses of Sinopharm vaccine with 28±5 days interval Receiving 2 doses of Astrazeneca vaccine with 3 to 4 months interval All candidates whose second vaccination either Sinopharm or Astrazeneca was done 3 to 6 prior to enrollment.

Signed informed written consent Able to follow the vaccination schedules, visits and tests General health and controlled underlying diseases (based on the physician's recommendation) Iranian citizenship Resident in Tehran Both sexes (male and female) Aged above 18 years Receiving 2 doses of Sinopharm vaccine with 28±5 days interval Receiving 2 doses of Astrazeneca vaccine with 3 to 4 months interval All candidates whose second vaccination either Sinopharm or Astrazeneca was done 3 to 6 prior to enrollment.