Booster mRNA Vaccination Prevents Breakthrough Severe COVID‐19 Infections

This study aimed to assess the humoral immune response following administration of a BNT162b2 mRNA COVID-19 booster dose and to evaluate its association with the development of SARS-CoV-2 infection after vaccination. Between July 2021 and February 2022, serum samples were collected from 313 individuals 28 days after receiving a COVID-19 booster dose. Quantitative SARS-CoV-2 IgG antibodies directed against the receptor-binding domain (RBD) of the spike protein were measured using a chemiluminescent microparticle immunoassay. Neutralizing antibody activity, defined as inhibition of the RBD–ACE2 interaction, was evaluated semi-quantitatively by a competitive ELISA. Participants were followed for 6 months after the final dose to identify confirmed SARS-CoV-2 infections. The cohort included 97 males (31%) and 216 females (69%), with a mean age of 39.43 years. The median SARS-CoV-2 IgG antibody level at day 28 was 7713.3 AU/mL, and the median neutralizing antibody inhibition rate was 99.2%. Antibody levels were not significantly associated with age or comorbidities. Booster vaccination induced a strong humoral immune response in all participants. However, the occurrence of post-vaccination infections despite high antibody levels suggests immune escape by emerging variants. While vaccines may not fully prevent infection, they provide substantial protection against severe disease and mortality.