1. Sororesponse is measured using anti-Spike IgG seroresponse rate (SRR)* in the fractional and full dose arms. SRR is defined as: 4-fold rise in GMTs at 28 days post study vaccine from baseline among participants with detectable Ab titers pre-booster; OR detectable Ab titres at 28 days post study vaccine among participants with no pre-dose detectable titres. 2. Safety and reactogenicity are measured using an electronic diary completed by participants in the 7 days following visit 1and up to 28 days if unsolicited adverse events occur. Solicited systemic adverse events include fever; chills; joint pains; muscle pains; fatigue; headache; nausea; and loss of appetite. Occurrence of SAEs and AESIs will be followed throughout the Trial.

1. Sororesponse is measured using anti-Spike IgG seroresponse rate (SRR)* in the fractional and full dose arms. SRR is defined as: 4-fold rise in GMTs at 28 days post study vaccine from baseline among participants with detectable Ab titers pre-booster; OR detectable Ab titres at 28 days post study vaccine among participants with no pre-dose detectable titres. 2. Safety and reactogenicity are measured using an electronic diary completed by participants in the 7 days following visit 1and up to 28 days if unsolicited adverse events occur. Solicited systemic adverse events include fever; chills; joint pains; muscle pains; fatigue; headache; nausea; and loss of appetite. Occurrence of SAEs and AESIs will be followed throughout the Trial.