Efficacy and Safety of SOBERANA 02, a COVID-19 conjugate vaccine in heterologous three doses combination

Background: SOBERANA 02 is a COVID19 conjugate vaccine (recombinant RBD conjugated to tetanus toxoid). Phase 1 and 2 clinical trials demonstrated its high immunogenicity, promoting neutralizing IgG together with specific T-cell response. A third dose of SOBERANA Plus (SARS-CoV-2 RBD-dimer) further increased the specific anti-RBD neutralizing antibodies. Methods: In a randomized, double-blinded, placebo-controlled, phase 3 trial we randomly assigned 44 031 participants, aged 19-80 years to three groups in a 1:1:1 ratio to receive 28 days apart either a) two doses of 25 microg SOBERANA 02, or b) two doses of 25 microg SOBERANA 02 followed by a third dose of 50 microg SOBERANA Plus, or c) two doses of placebo. Reported study endpoints are vaccine efficacy (VE) evaluated through laboratory-confirmed symptomatic COVID-19 cases and safety. During the trial, the SARS CoV-2 isolates in Havana were predominantly (beta 74.0 %) and shift gradually to delta (100%). Results: Two doses of SOBERANA 02 protects against symptomatic COVID-19: 43 cases in the two-dose group (14 371) vs. 155 in the placebo group (14 403), VE 71.0%, adjusted (CI 95%58.9-79.1). The heterologous three dose combination with SOBERANA Plus protected against symptomatic COVID-19: 15 cases in the vaccine groups (13 833) vs. 155 in the placebo group (14 303), VE 92.4%, adjusted (CI 95% 86.9-95.6%). For two-dose schedule VE against severe COVID-19 was 63.0% and for death 59.0%; for heterologous three-dose schedule, 100% in both cases. Conclusions: This is the first phase 3 study of a three-dose, heterologous vaccine combination against SARS-CoV-2. Two doses of the conjugate vaccine SOBERANA 02 was safe and attained efficacy of 71.0% in adults population 19-80 y/o; incorporating SOBERANA Plus after two doses of SOBERANA 02, increased efficacy from 71.0 % to 92.4% (Clinical Trials IFV/COR/09 number, RPCEC00000354.)