Longitudinal Study of IgG Antibody Levels Over 4.5 Years Until the Fifth Booster mRNA Vaccination and Breakthrough Infection: A Case Report

The differential antibody responses and persistence of immunity following booster vaccinations and breakthrough infections with Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) remain inadequately understood. This case study examined a 71-year-old Japanese man who received his seventh dose (fifth booster) of the mRNA BNT162b2 vaccine and subsequently experienced his first breakthrough infection 80 days after vaccination. A total of twenty-three serological assays were performed to quantify anti-spike protein immunoglobulin G (IgG) antibody levels from one week to one year and eight months post-infection. Six weeks after infection, the antibody titer peaked at 76,000 AU/ml, which was 14 times higher than the baseline level (5,555 AU/ml). At one year and eight months post-infection, the titer remained at 19,000 AU/ml, representing 3.8 times the baseline and one-fourth of the peak value observed after the sixth vaccination. The decline in antibody titers after breakthrough infection was modeled using the equation f(t) = Aexp(−t/τ) + C. These findings suggest that humoral immune responses are maintained at elevated levels for an extended period. This case provides valuable insights into vaccination strategies for breakthrough infections.