Fig. 6. Persistence of neutralizing antibodies and immune protection.

a Immunization and challenge schedule for IFNAR−/− mice (n = 6 and n = 4 in the control group; Empty MV). Animals were immunized interperitoneally by homologous prime-boost at days 0 and 28. Sera were collected at days 52, 72, and 110. Animals were challenged on day 110 by intranasal inoculation of mouse-adapted SARS-CoV-2 virus (MACo3) at 1.5 × 10⁵ PFU. Sera were assessed for levels of specific antibodies against b MV and c SARS-CoV-2 S. d Neutralizing antibody responses against SARS-CoV-2 virus, expressed as 50% plaque reduction neutralization test (PRNT₅₀) titers. e SARS-CoV-2 viral RNA copies detected by RT-qPCR in homogenized lungs of challenged animals, calculated as copies/lung. f Titer of infectious viral particles recovered from the homogenized lung of the immunized animals expressed as PFU/lung. Dotted blue line indicates the limit of detection (LOD). Data are represented as geometric means with line and error bars indicating ±geometric SD. Statistical significance was determined by (c, d) a two-way ANOVA with Tukey’s multiple comparisons test and (e) two-tailed the Mann–Whitney test (f) Kruskal–Wallis one-way ANOVA test with Dunn’s multiple comparison test.