This study, a pilot randomized controlled trial, single-blind (outcome assessor), evaluating the immune response (both humoral and cell-mediated) against SARS-CoV-2 and the safety of three different vaccine administration approaches: the SARS-CoV-2 mRNA vaccine alone, the influenza vaccine alone, and the co-administration of both vaccines.

Inclusion Criteria:

• Thai adults aged 18-60 years, who have vaccinated COVID-19 and influenza vaccine more than 6 months.
• The subjects are able to and willing to comply with the requirements of the clinical trial program and could complete the 1-month follow-up of the study.
• Individuals who are in good health condition at the time of entry into the trial as determined by medical history, physical examination and clinical judgment of the investigator and meet the requirements of immunization.
• The subject can provide with informed consent and sign informed consent form (ICF).

Exclusion Criteria:

• Have history of influenza or COVID-19 infection within 6 months.
• Have the medical history or family history of convulsion, epilepsy, encephalopathy, and psychosis.
• Be allergic to any component of the research vaccines or used to have a history of hypersensitivity or serious reactions to vaccination.
• History of Guillain-Barré syndrome.
• Women with positive urine pregnancy test, pregnant or breast-feeding, or have a pregnancy plan within six months of after baseline period.
• Have infectious diseases, including HIV and SARS-CoV-2 infection or any symptom of common cold.
• Have history of SARS-CoV-2 infection less than 6 months.
• Having severe chronic diseases or condition in progress cannot be controlled. For example, poor controlled DM and uncontrolled HT.
• Have the history of urticaria 1 year before receiving the investigational vaccine.
• Have known underlying diseases of thrombocytopenia or other coagulation disorders (which may cause contraindications for intramuscular injection).
• Having needle sickness.
• Have the history of immunosuppressive therapy, cytotoxic therapy, or systemic corticosteroids.
• Have received blood products within 4 months before injection of investigational vaccines.
• Under anti-tuberculosis treatment.
• Not be able to follow the protocol, or not be able to understand the informed consent according to the researcher's judgment, due to various medical, psychological, social, or other conditions.

The data will be kept anonymized.

From vaccination until 28 days post vaccination

Pre-specified adverse events included solicited local and systemic reactions, assessed at Day 7 and Day 28 post-vaccination. Unsolicited adverse events were monitored daily from Day 1 through Day 28. Serious adverse events were monitored throughout Days 1-28.

As a single-center pilot study with a limited number of participants, the findings may not be generalizable. The short follow-up duration also restricts conclusions about long-term safety and immune responses.