A 42-year-old woman from Mumbai, who is fully vaccinated with 2 doses of inactivated SARS-CoV-2 vaccine (completed 8 months ago), presents with fever (38.5°C), cough, and mild dyspnea for 3 days. She has no comorbidities. On examination, SpO₂ is 94% on room air, and chest X-ray shows minimal infiltrates. RT-PCR confirms SARS-CoV-2 (Omicron variant). Her serum antibody levels are: IgG anti-spike 450 AU/mL (reference >50 = seropositive), IgM anti-spike negative. Which of the following best explains her clinical presentation and serological pattern?