A SARS-CoV-2 recombinant spike protein vaccine (S-268019-b) for COVID-19 prevention during the Omicron-dominant period: A phase 3, randomised, placebo-controlled clinical trial

Clinical trials of new vaccines based on existing variants of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) are often impacted by the emergence of new virus variants. We evaluated the efficacy, immunogenicity, and safety of S -268019-b, a recombinant spike protein subunit vaccine based on the ancestral strain, for preventing symptomatic coronavirus disease 2019 (COVID-19) during the Omicron (BA.2)-dominant period in Vietnam. In this multicentre, phase 3, randomised (2:1), observer -blind, placebo -controlled crossover study, participants received 2 intramuscular doses (28 days apart) of either 10 mu g of S -268019-b (Recombinant Sprotein vaccine) or placebo. The primary endpoint was incidence of laboratory -confirmed symptomatic COVID19 before crossover, with onset within 14 days following the second dose, in participants who were seronegative and reverse transcription polymerase chain reaction (RT-PCR)-negative at baseline. The secondary endpoints included immunogenicity and safety. In total, 8,594 participants were randomised (S -268019-b [n = 5,727]; placebo [n = 2,867]). Vaccine efficacy versus placebo was 39.1 % (95 % confidence interval [CI]:26.6-49.5; onesided P = 0.0723). The incidence rate (95 % CI) of symptomatic COVID-19 was 776.41/1,000 person -years (682.04-880.19) in the S -268019-b group and 1272.87/1,000 person -years (1101.32-1463.57) in the placebo group. The geometric mean titres (95 % CI) of the SARS-CoV-2 neutralising antibody increased on Day 57 versus baseline with S -268019-b (34.66 [27.04-44.41] versus 2.50 (non -estimable) but not with placebo. There were no safety concerns regarding S -268019-b. S -268019-b did not demonstrate the targeted efficacy threshold against symptomatic COVID-19; however, findings were comparable with other prophylactic vaccines based on ancestor strain during the Omicron -dominant period. S -268019-b demonstrated immunogenicity and was well -tolerated. ClinicalTrials.gov identifier: NCT05212948.