A binary response, whereby a participant with negative serostatus at baseline is defined as a COVID-19 case if their first case of SARS-CoV-2 RT-PCR-positive symptomatic illness occurs ≥ 15 days post second dose of study intervention. Otherwise, a participant is not defined as a COVID-19 case. The primary efficacy analysis was performed once approximately 150 events meeting the primary efficacy outcome measure definition had occurred across the AZD1222 and placebo groups.

An AE is the development of any untoward medical occurrence in a clinical study participant administered medicinal product and which does not necessarily have a causal relationship with this medicinal product. An AE can therefore be any unfavorable and unintended sign, symptom, or disease temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product.

An SAE is an AE occurring during any study phase that fulfils 1 or more of the following criteria: death; immediately life-threatening; in-participant hospitalization or prolongation of existing hospitalization; persistent or significant disability or incapacity; congenital abnormality or birth defect; an important medical event. AESIs were events of scientific and medical interest specific to the further understanding of the study intervention safety profile and required close monitoring and rapid communication by the investigators to the sponsor. MAAEs are defined as AEs leading to medically-attended visits that were not routine visits for physical examination or vaccination, such as an emergency room visit, or an otherwise unscheduled visit to or from medical personnel (medical doctor) for any reason. Different follow-up time between AZD1222 and Placebo groups (20223 versus 3893 participant years).

Solicited AEs are local or systemic predefined events for assessment of reactogenicity. Solicited AEs were collected in a e-Diary only for participants in the substudy.

Antibody seroconversion rate (≥4-fold increase from baseline) to SARS CoV-2 Spike protein 29 days after the second vaccination.

An adverse event (AE) is any untoward medical occurrence in a participant who received study drug without regard to possibility of causal relationship. The TEAEs are defined as events present at baseline that worsened in intensity after administration of study drug or events absent at baseline that emerged after administration of study drug. Solicited AEs are local or systemic predefined events for assessment of reactogenicity. An e-diary was used to collect information on the timing and severity of the solicited AEs. Local AEs included pain, redness/erythema, tenderness, induration/swelling at the site of the injection. Systemic AEs included fever (\> 100 °F/37.8 °C), chills, muscle pains, fatigue, headache, malaise, nausea, and vomiting.

The AEs other than solicited AEs are reported as unsolicited AEs and were collected by "open question" at study visits. AE: any untoward medical occurrence in participant who received study drug without regard to possibility of causal relationship. TEAEs: events present at baseline that worsened in intensity after administration of study drug or events absent at baseline that emerged after administration of study drug. SAE: an AE resulting in any of following outcomes or deemed significant for any other reason: death; initial or prolonged inpatient hospitalization; life threatening experience; persistent or significant disability/incapacity; congenital anomaly. MAAE: an AE leading to a non-routine/unscheduled medically-attended visit, to or from medical doctor for any reason. AESI: an event of scientific and medical interest specific to further understanding of study drug safety profile and require close monitoring and rapid communication by investigators to Sponsor.

Number of participants with abnormal laboratory parameters reported as TEAEs are reported. Laboratory tests included haematology and clinical chemistry.

Severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) nAb were measured by pseudoneutralisation assay. The GMT was calculated as the antilogarithm of Σ(log base 2 transformed titre/n), i.e. as the anti-logarithm transformation of the mean of the log-transformed titre, where 'n' was the number of participants with titre information. Primary Vaccination Cohort:- AZD2816 (4) with response against B.1.351 variant is the comparator group and Primary Vaccination Cohort:- AZD1222 (4) with response against the original Wuhan-Hu-1 strain is the reference group.

Severe acute respiratory syndrome-coronavirus-2 nAb were measured by pseudoneutralisation assay. The GMT was calculated as the antilogarithm of Σ(log base 2 transformed titre/n), i.e. as the anti-logarithm transformation of the mean of the log-transformed titre, where 'n' was the number of participants with titre information. Booster Cohort:- AZD1222:AZD1222 is the comparator group and AZD1222 in Historical Control is the reference group, both compared for response against the original Wuhan-Hu-1 strain.

Severe acute respiratory syndrome-coronavirus-2 nAb were measured by pseudoneutralisation assay. Seroresponse was defined as \>= 4-fold increase in the GMT of nAb from baseline. Primary Vaccination Cohort:- AZD2816 (4) with response against B.1.351 variant is the comparator group and Primary Vaccination Cohort:- AZD1222 (4) with response against the original Wuhan-Hu-1 strain is the reference group.

Severe acute respiratory syndrome-coronavirus-2 nAb were measured by pseudoneutralisation assay. The GMT was calculated as the antilogarithm of Σ(log base 2 transformed titre/n), i.e. as the anti-logarithm transformation of the mean of the log-transformed titre, where 'n' was the number of participants with titre information. Booster Cohort:- mRNA:AZD1222 is the comparator group and AZD1222 in Historical Control is the reference group, both compared for response against the original Wuhan-Hu-1 strain.

Seroresponse is a binary outcome where a success is when the fold rise in titers compared with baseline is \>= 4. The fold rise in titers was calculated as the ratio of the post-vaccination titer level to the baseline titer level. The percentage of participants with a post-vaccination seroresponse (\>= 4-fold rise in titers from Day 1 baseline value to Day 57) to the S antigen of AZD1222 as measured by MSD serology assay is reported.

Solicited AEs are local or systemic predefined AEs for reactogenicity assessment. Participants were given an axillary thermometer, tape measure, and access to app for the solicited AE eDiary, with instructions on use, along with the emergency 24-hour telephone number to contact the on-call study physician if needed. Participants were instructed to record for 7 days following administration of each dose of AZD1222, the timing and severity of local and systemic solicited AEs, if applicable, and whether medication was taken to relieve the symptoms.

Solicited AEs are local or systemic predefined AEs for reactogenicity assessment. Participants were given an axillary thermometer, tape measure, and access to app for the solicited AE eDiary, with instructions on use, along with the emergency 24-hour telephone number to contact the on-call study physician if needed. Participants were instructed to record for 7 days following administration of each dose of AZD1222, the timing and severity of local and systemic solicited AEs, if applicable, and whether medication was taken to relieve the symptoms.

An AE is the development of any untoward medical occurrence in a clinical study participant administered medicinal product and which does not necessarily have a causal relationship with this medicinal product. An AE can therefore be any unfavorable and unintended sign, symptom, or disease temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product. An SAE is an AE occurring during any study phase that fulfils one or more of the following criteria: death; immediately life-threatening; in-participant hospitalization or prolongation of existing hospitalization; persistent or significant disability or incapacity; congenital abnormality or birth defect; an important medical event. The AESIs were events of scientific and medical interest specific to the further understanding of the study vaccination safety profile and required close monitoring and rapid communication by the investigators to the sponsor.

Clinical laboratory values were evaluated for each laboratory parameter as applicable including hematology and clinical chemistry. The baseline was defined as the last non-missing measurement taken prior to the first dose of study vaccination (including unscheduled measurements, if any).