Intranasal immunization with the recombinant measles virus encoding the spike protein of SARS-CoV-2 confers protective immunity against COVID-19 in hamsters

Background: As the nasal mucosa is the initial site of infection for COVID-19, intranasal vaccines are more favorable than conventional vaccines. In recent clinical studies, intranasal immunization has been shown to generate higher neutralizing antibodies; however, there is a lack of evidence on sterilizing immunity in the upper airway. Previously, we developed a recombinant measles virus encoding the spike protein of SARS-CoV-2 (rMeV-S), eliciting humoral and cellular immune responses against SARS-CoV-2. Objectives: In this study, we aim to provide an experiment on nasal vaccines focusing on a measles virus platform as well as injection routes. Study design: Recombinant measles viruses expressing rMeV-S were prepared, and 5 × 10⁵ PFUs of rMeV-S were administered to Syrian golden hamsters via intramuscular or intranasal injection. Subsequently, the hamsters were challenged with inoculations of 1 × 10⁵ PFUs of SARS-CoV-2 and euthanized 4 days post-infection. Neutralizing antibodies and RBD-specific IgG in the serum and RBD-specific IgA in the bronchoalveolar lavage fluid (BALF) were measured, and SARS-CoV-2 clearance capacity was determined via quantitative reverse-transcription PCR (qRT-PCR) analysis and viral titer measurement in the upper respiratory tract and lungs. Immunohistochemistry and histopathological examinations of lung samples from experimental hamsters were conducted. Results: The intranasal immunization of rMeV-S elicits protective immune responses and alleviates virus-induced pathophysiology, such as body weight reduction and lung weight increase in hamsters. Furthermore, lung immunohistochemistry demonstrated that intranasal rMeV-S immunization induces effective SARS-CoV-2 clearance that correlates with viral RNA content, as determined by qRT-PCR, in the lung and nasal wash samples, SARS-CoV-2 viral titers in lung, nasal wash, BALF samples, serum RBD-specific IgG concentration, and RBD-specific IgA concentration in the BALF. Conclusion: An intranasal vaccine based on the measles virus platform is a promising strategy owing to the typical route of infection of the virus, the ease of administration of the vaccine, and the strong immune response it elicits.