This is a Phase II study with single-blinded safety phase followed by double-blinded randomization, placebo-controlled, of administration of a single dose by two different administration routes (intramuscular route or intranasal route), to evaluate immunogenicity and safety of the recombinant SARS-CoV-2 vaccine (AVX/COVID-12 vaccine) based a live Newcastle disease viral vector (rNDV) in 396 healthy subjects with evidence of prior immunity to SARS-CoV-2, followed by a booster response assessment with an intramuscular dose of COVID-19 vaccine (ChAdOx-1 -S[recombinant]) in subjects originally randomized to the placebo arm at several research sites in Mexico City.

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.

Inclusion Criteria:

1. Be ≥ 18 years old.

2. Indistinct sex.

3. Having given their informed consent.

4. No respiratory problems during the last 21 days prior to administration of the single dose.

5. No conditions or alterations in the physical examination, laboratory values and cabinet that in the opinion of the investigator may interfere with the participation of the subject in the study or require a more detailed medical study.

6. Negative PCR test for SARS-CoV-2 during the screening visit.

7. Negative pregnancy test in women with pregnancy potential.

8. Signature of commitment for the use of highly effective contraceptive methods for at least 30 days after administration of the intramuscular injection or intranasal.

9. In case of presenting any chronic disease with medical management, it must be controlled and stable without changes in treatment during the last three months prior to the scrutiny visit.

10. Commitment to maintain adequate prevention measures to avoid the contagion by SARS-CoV-2 during their participation in the study, considering themselves these strict use during the first 14 days after the baseline visit (Use of face masks in closed places, social distancing measures in spaces open, and frequent hand washing).

11. Present detectable titers of anti-Spike IgG in peripheral serum during the visit of screening with titers less than 1,200 U/mL in a chemiluminescence test.

12. Submit proof of vaccination 4 months or more after the last vaccination

13. Have been vaccinated with the complete program of any of the following vaccines against SARS-CoV-2:

    • ModeRNA
    • Pfizer
    • AstraZeneca
    • CanSino
    • Sinovac
    • Sinopharm
    • Johnson & Johnson (Janssen)
    • Sputnik V

Exclusion Criteria:

1. History of hypersensitivity or allergy to any of the components of the vaccine.
2. History of severe anaphylactic reactions from any cause.
3. History of seizures.
4. Uncontrolled chronic diseases.
5. Chronic diseases that require management with immunosuppressive agents or immune response modulators (eg, systemic corticosteroids, cyclosporine, rituximab among others).
6. Oncological disease.
7. Active participation, or during the last 3 months in any other clinical study or research experimental intervention.
8. Use within 30 days prior to screening evaluation of any drug or herbal supplement, or alternative medicine (for example, transfer factor, chlorine dioxide, etc.) aimed at treating or preventing complications or contagion by SARS-CoV-2, or any other condition.
9. Febrile illness at the time of the screening visit.
10. Have received any vaccine (experimental or approved) during the 60 days prior to the scrutiny visit.
11. Having received a blood transfusion or blood components during the last 4 months prior to the scrutiny hearing.
12. Have been a plasma donor during the last 4 months prior to the visit of scrutiny.
13. Have undergone dialysis or hemodialysis procedures during the last year prior to the scrutiny visit.
14. Work on poultry or gamecock farms.
15. History of substance abuse problems that in the opinion of the investigator could interfere with the subject's ability to adequately comply with the protocol guidelines.

Individual deidentified participant data (including data dictionaries) will not be shared, but only to the extent permitted in the informed consent and under Mexican law.

The data will be available immediately following publication and for 12 months thereafter.

Data will be available only to investigators whose proposed use of the data has been authorized by an independent review committee and the ethics committees involved in the authorization of the protocol, and/or the Federal Commission for the Protection against Sanitary Risks (COFEPRIS) in Mexico if required by law. The permitted use of the data will be as authorized. Proposals or requests should be directed to gustavo.peralta@avimex.com.mx and authorized data requestors must sign a data access agreement.

The first sentinel group (6 of each previous vaccine) will be without masked only to the subjects to ensure safety measurement in each group. The randomized double-mask placebo-controlled phase will begin when the safety committee, after evaluating the information from the sentinel group of the first three subjects (by vaccine and by specific route of administration), gives authorization to continue with the recruitment. It will be conducted through a computerized assignment system. Randomization for the double-masking phase of the study will be carried out using a computer assignment system.