The fourth COVID-19 vaccine dose increased the neutralizing antibody response against the SARS-CoV-2 Omicron (B.1.1.529) variant in a diverse Brazilian population.

IMPORTANCE: Several additional COVID-19 vaccine doses were administered in the Brazilian population to prevent the disease caused by the B.1.1.529 (Omicron) variant. The efficacy of a third dose as a booster is already well described. However, it is important to clarify the humoral immune response gain induced by a fourth dose. In this study, we evaluate the effect of the fourth COVID-19 vaccine dose in a diverse Brazilian population, considering a real-life context. Our study reveals that the fourth dose of the COVID-19 vaccine increased the neutralizing antibody response against SARS-CoV-2 Omicron and significantly contributed in the reduction of the disease caused by this variant.