Intranasal inoculation of an MVA-based vaccine induces IgA and protects the respiratory tract of hACE2 mice from SARS-CoV-2 infection

Current vaccines have greatly diminished the severity of the COVID-19 pandemic, even though they do not entirely prevent infection and transmission, likely due to insufficient immunity in the upper respiratory tract. Here, we compare intramuscular and intranasal administration of a live, replication-deficient modified vaccinia virus Ankara (MVA)–based Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) spike (S) vaccine to raise protective immune responses in the K18-hACE2 mouse model. Using a recombinant MVA expressing firefly luciferase for tracking, live imaging revealed luminescence of the respiratory tract of mice within 6 h and persisting for 3 d following intranasal inoculation, whereas luminescence remained at the site of intramuscular vaccination. Intramuscular vaccination induced S-binding–Immunoglobulin G (IgG) and neutralizing antibodies in the lungs, whereas intranasal vaccination also induced Immunoglobulin A (IgA) and higher levels of antigen-specific CD3⁺CD8⁺IFN-γ⁺ T cells. Similarly, IgG and neutralizing antibodies were present in the blood of mice immunized intranasally and intramuscularly, but IgA was detected only after intranasal inoculation. Intranasal boosting increased IgA after intranasal or intramuscular priming. While intramuscular vaccination prevented morbidity and cleared SARS-CoV-2 from the respiratory tract within several days after challenge, intranasal vaccination was more effective as neither infectious virus nor viral messenger (m)RNAs were detected in the nasal turbinates or lungs as early as 2 d after challenge, indicating prevention or rapid elimination of SARS-CoV-2 infection. Additionally, we determined that neutralizing antibody persisted for more than 6 mo and that serum induced to the Wuhan S protein neutralized pseudoviruses expressing the S proteins of variants, although with less potency, particularly for Beta and Omicron.

The rapid development of SARS-CoV-2 vaccines was a stunning achievement that is contributing to the control of the COVID-19 pandemic. Several types of vaccines—including mRNA, adenovirus-vectors, recombinant spike (S) protein, and inactivated Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2)—have demonstrated the ability to protect against severe disease. Nevertheless, these vaccines, which are administered systemically, reduce but do not prevent virus infection and transmission, and therefore approaches that provide further immunity are desirable (1). SARS-CoV-2 spreads by droplet and aerosol so that the nasal and oral mucosa are the first barriers to infection. In general, the intranasal (IN) route of vaccination induces greater mucosal immunity compared with the intramuscular (IM) route. An example is the live, attenuated influenza virus vaccine, called LAIV or FluMist, which is approved as a nasal spray in some countries. Unlike inactivated influenza vaccine, LAIV induces nasal Immunoglobulin A (IgA) and CD8⁺ T cells (2). Similarly, IN administration of adenovirus-vectored SARS-CoV-2 vaccines reduce viral loads in upper and lower respiratory tracts following challenge in several animal models (3–6) and an aerosolized vaccine appeared safe and immunogenic in a phase I trial (7), although a trial of another adenovirus-based nasal spray vaccine was discontinued because of low immunogenicity (https://ir.altimmune.com/news-releases/news-release-details/altimmune-announces-update-adcovidtm-phase-1-clinical-trial). Studies of IN vaccination with additional vectors are needed.Modified vaccinia virus Ankara (MVA) is a highly attenuated, replication-defective, immunogenic smallpox vaccine strain that is undergoing clinical testing as a vector for multiple pathogens (8) as well as SARS-CoV-2 (www.clinical trials.gov). Although usually administered IM or subcutaneously, several reports have shown that MVA-based vectors induce protective mucosal and systemic immune responses when administered IN to animals (9–13). In addition, combined IM and IN vaccination of camels with an MVA-based vaccine reduced excretion of Middle East respiratory syndrome (MERS)-CoV, although the efficacy of IN alone was not reported (14).The present study was initiated to extend previous demonstrations of the ability of IM administered MVA-vectored vaccines to protect against SARS-CoV-2 challenge in animal models (15–18). We previously reported (15) that IM injection of MVA expressing a modified S protein with mutations that stabilized the prefusion form, inactivated the furin cleavage site, and deleted the endoplasmic retention signal induced a type 1 immune response with neutralizing antibody and CD8⁺IFN-γ⁺ T cells, and protected K18-hACE2 transgenic mice from respiratory infection with SARS-CoV-2. In addition, passive transfer of serum from vaccinated mice to unvaccinated mice protected them from lethal SARS-CoV-2 infection. Here, we show persistence of neutralizing antibody and protection of transgenic hACE2 mice for more than 6 mo after one or two IM inoculations with an MVA-based modified S protein vaccine. However, whereas IM vaccination induced Immunoglobulin G (IgG) neutralizing antibodies and cleared infection of the respiratory tract, IN inoculations also induced IgA antibodies in the lungs and blood, and after two IN vaccinations neither SARS-CoV-2 nor subgenomic (sg) mRNAs were detected in the nasal turbinates or lungs at 2 or 5 d after challenge. IN delivery of a live recombinant vaccine has the potential to reduce infection and transmission of SARS-CoV-2.