Phase II Study of AVX/COVID-12 Vaccine in Subjects With Prior SARS-CoV-2 Immunity Evidence

General objective: To demonstrate immunogenicity due to the administration of a single dose of AVX/COVID-12 vaccine at a dose of 108.0 EID50%/dose by the intramuscular or intranasal route in subjects with evidence of prior immunity to SARS-CoV-2. Primary objectives: * To demonstrate an increase in the total titers of neutralizing anti-Spike IgG antibodies in serum, as well as an increase in the proportion of peripheral blood T lymphocytes that produce interferon gamma in response to stimulation with Spike protein or peptides derived from Spike protein, 14 days after intramuscular administration of a single dose of AVX/COVID-12 vaccine (108.0 EID50%/dose) compared to the response obtained after administration of placebo. * To demonstrate an increase in the total titers of neutralizing anti-Spike IgG antibodies in serum, as well as an increase in the proportion of peripheral blood T lymphocytes that produce interferon gamma in response to stimulation with Spike protein or peptides derived from Spike protein, 14 days after intranasal administration of a single dose of AVX/COVID-12 vaccine (108.0 EID50%/dose) compared with the response obtained after administration of placebo. Secondary objectives: * To evaluate serum titers of IgG anti-Spike antibodies and neutralizing anti-SARS-CoV-2 antibodies at 0, 14, 42, 90, 180 and 365 days after intramuscular or intranasal administration of the AVX/COVID-12 vaccine. * To assess the proportion of proliferating cells and cytokine production by peripheral blood T lymphocytes in response to stimulation with Spike protein at 0, 14, 180 and 365 days after intramuscular or intranasal administration of the AVX vaccine/COVID-12. Safety objective: To assess the safety of immunization using a single intramuscularly or intranasally administered dose of AVX/COVID-12 vaccine in subjects with prior immunity to SARS-CoV-2. Exploratory objectives: • Evaluation of the increase in the immune response according to the previously defined parameters of anti-Spike IgG antibody titers, serum titers of neutralizing anti-SARS-CoV-2 antibodies, the increase in the proportion of T lymphocytes producing Gamma interferon according to two stratification schemes as long as the number of recruited subjects allows it: A) By underlying technology of the vaccines received prior to enrollment in the study. The three groups of technology to be explored are inactivated viruses, adenoviral vectors, and mRNA-based technology. B) By specific vaccine received prior to enrollment in the study as long as the number of subjects recruited for each vaccine allows it. • Evaluation of the local response in nasal mucous for subjects who were randomized to receive vaccination by both routes, which consists of: A) The determination and quantification of IgA anti-Spike antibodies in mucus or epithelial scraping samples or in an oral fluid rinse. * Evaluation of the appearance of anti-N and anti-S antibodies on days -4, 7, 14, 42, 90, 180 and 365 days after intramuscular or intranasal administration of the AVX/COVID-12 vaccine. * Evaluation of the incidence of confirmed cases of SARS-CoV-2 infection in study subjects starting from vaccination in a systematic way until day 28 post-vaccination and in a targeted way (in case of suspicious symptoms) until the end of the study. * Evaluation of the increase in the immune response according to the parameters of anti-Spike IgG-type antibody titers, anti-SARS-CoV-2 neutralizing antibody serum titers and an increase in the proportion of interferon gamma-producing T lymphocytes in subjects who are originally randomized to placebo and subsequently receive an intramuscular boost with COVID-19 vaccine ChAdOx-1-S\[recombinant\]). Clinical trial hypothesis: Intramuscular or intranasal administration of the AVX/COVID-12 vaccine at a dose of 108.0 EID50%/dose in subjects with prior immunity to SARS-CoV-2 (induced by vaccination) produces an increase in total serum antibody titers of type IgG anti-Spike and increases the titers of neutralizing anti-SARS-CoV-2 antibodies, and additionally produces an increase in the proportion of interferon gamma-producing T lymphocytes in response to stimulation with the Spike protein or peptides derived from the Spike protein, when these parameters are analyzed 14 days after administration. Rationale of the use of the product in clinical research: Non-clinical studies and the phase I clinical trial have demonstrated the safety of the AVX/COVID-12 vaccine in intramuscular as well as intranasal route of administration. The signals of immunogenicity in animal models are clear. The evaluation of the immune response in healthy volunteers during the Phase I clinical study after the administration of the vaccine either intramuscularly or intranasally demonstrated the immunogenicity of the vaccine, which justifies the advance of the vaccine development program. Clinical trial design: A mixed with single-blind low safety phase followed by double-blind randomization, placebo-controlled, single-dose intramuscular or intranasal, phase II study, in subjects with evidence of prior immunity to SARS-CoV-2, followed by a booster response assessment phase with an intramuscular dose of COVID-19 vaccine (ChAdOx-1-S\[recombinant\]) in subjects originall randomized to the placebo arm. . Description of the single-blind safety phases and the double-blind randomization placebo-controlled phase and the evaluation phase of the response to the booster with the COVID-10 vaccine (ChAdOx-1-S\[recombinant\]): For each arm of the study (intramuscular or intranasal): A) The safety phase consists of the single-blind assignment (subject blinded) to the arm that receives the AVX/COVID-12 vaccine to the first three subjects of each specific vaccine who are recruited to the study in each arm, intramuscular and intranasal, independently. The first three subjects in each arm will be subjected to a strict safety follow-up during the 7 days after the administration of the vaccine. The safety information collected during those seven days will be evaluated by an independent safety committee who will decide if it is necessary to stop the study of the specific vaccine for safety reasons or if it is possible to continue with the recruitment of the subjects of the corresponding vaccine evaluated. B) The randomized, double-blind, placebo-controlled phase will begin when the safety committee, after having evaluated the information of the sentinel group of the first three subjects (by vaccine and by specific route of administration), gives the authorization to continue with the recruitment. From that moment on, the subjects with the corresponding vaccination history will be randomized to receive placebo or the AVX/COVID-12 vaccine in either of the two administration arms (intramuscular or intranasal) in such a way that the total number of subjects recruited of the study for this vaccine are distributed as close as possible to 1:1 vaccine: placebo, and in equilibrium between the intramuscular and intranasal arms. C) Specific recruitment considerations (see also sample size calculation below): To allow exploratory analyzes to be carried out for each route to be tested, by vaccine technology group or by vaccine, guiding objectives (not absolute) will be for the recruitment of 66 subjects with a history of vaccination with adenoviral technologies, 66 subjects with mRNA technologies and 66 subjects with inactivated virus technologies. D) At the end of the study period necessary to meet the primary efficacy criterion of the study (14 days after administration of the AVX/COVID-12 vaccine or placebo), the subjects who received placebo will be masked. These subjects will receive an additional dose of the COVID-19 vaccine (ChAdOx-1-S\[recombinant\]) intramuscularly, and immune response parameters will be reassessed in the same manner as during the double-blind phase of the study. From that moment on, all follow-ups will continue as for the arm that originally received the AVX/COVID-12 vaccine.