inclusion criteria: participant is willing and able to give written informed consent for participation in the trial. male or female, aged from 20 to 70 years has received one dose of the azd1222 within 28-70 days before randomization. evidence of this will be gathered from medical history and/or medical records including the covid-19 vaccine registration yellow card. female participant must: be either of non-childbearing potential, i.e. surgically sterilized (defined as having undergone hysterectomy and/or bilateral oophorectomy and/or bilateral salpingectomy; tubal ligation alone is not considered sufficient) or one year post-menopausal; or, if of childbearing potential, be abstinent or agree to use medically effective contraception on enrolment continuously until 90 days after boost immunization of study intervention. acceptable forms include: i. implanted hormonal methods of contraception or placement of an intrauterine device or intrauterine system ii. established use of hormonal methods (injectable, pill, patch or ring) combined with barrier methods of contraception: condom or occlusive cap (diaphragm or cervical/vault caps) with spermicidal foam/gel/film/cream/suppository c. have a negative pregnancy test. in the investigator's opinion, is able and willing to comply with all trial requirements.

inclusion criteria: participant is willing and able to give written informed consent for participation in the trial. male or female, aged from 20 to 70 years has received one dose of the azd1222 within 28-70 days before randomization. evidence of this will be gathered from medical history and/or medical records including the covid-19 vaccine registration yellow card. female participant must: be either of non-childbearing potential, i.e. surgically sterilized (defined as having undergone hysterectomy and/or bilateral oophorectomy and/or bilateral salpingectomy; tubal ligation alone is not considered sufficient) or one year post-menopausal; or, if of childbearing potential, be abstinent or agree to use medically effective contraception on enrolment continuously until 90 days after boost immunization of study intervention. acceptable forms include: i. implanted hormonal methods of contraception or placement of an intrauterine device or intrauterine system ii. established use of hormonal methods (injectable, pill, patch or ring) combined with barrier methods of contraception: condom or occlusive cap (diaphragm or cervical/vault caps) with spermicidal foam/gel/film/cream/suppository c. have a negative pregnancy test. in the investigator's opinion, is able and willing to comply with all trial requirements.

inclusion criteria: 1. participant is willing and able to give written informed consent for participation in the trial. 2. male or female, aged from 20 to 70 years 3. has received one dose of the azd1222 within 28-70 days before randomization. evidence of this will be gathered from medical history and/or medical records including the covid-19 vaccine registration yellow card. 4. female participant must: 1. be either of non-childbearing potential, i.e. surgically sterilized (defined as having undergone hysterectomy and/or bilateral oophorectomy and/or bilateral salpingectomy; tubal ligation alone is not considered sufficient) or one year post-menopausal; 2. or, if of childbearing potential, be abstinent or agree to use medically effective contraception on enrolment continuously until 90 days after boost immunization of study intervention. acceptable forms include: i. implanted hormonal methods of contraception or placement of an intrauterine device or intrauterine system ii. established use of hormonal methods (injectable, pill, patch or ring) combined with barrier methods of contraception: condom or occlusive cap (diaphragm or cervical/vault caps) with spermicidal foam/gel/film/cream/suppository c. have a negative pregnancy test. 5. in the investigator's opinion, is able and willing to comply with all trial requirements.

inclusion criteria: 1. participant is willing and able to give written informed consent for participation in the trial. 2. male or female, aged from 20 to 70 years 3. has received one dose of the azd1222 within 28-70 days before randomization. evidence of this will be gathered from medical history and/or medical records including the covid-19 vaccine registration yellow card. 4. female participant must: 1. be either of non-childbearing potential, i.e. surgically sterilized (defined as having undergone hysterectomy and/or bilateral oophorectomy and/or bilateral salpingectomy; tubal ligation alone is not considered sufficient) or one year post-menopausal; 2. or, if of childbearing potential, be abstinent or agree to use medically effective contraception on enrolment continuously until 90 days after boost immunization of study intervention. acceptable forms include: i. implanted hormonal methods of contraception or placement of an intrauterine device or intrauterine system ii. established use of hormonal methods (injectable, pill, patch or ring) combined with barrier methods of contraception: condom or occlusive cap (diaphragm or cervical/vault caps) with spermicidal foam/gel/film/cream/suppository c. have a negative pregnancy test. 5. in the investigator's opinion, is able and willing to comply with all trial requirements.