INTRODUCTION
The SARS-CoV-2 coronavirus pandemic (Zhu et al., 2020) initiated a scientific race for the development of vaccines (Graham, 2020). The most widely used vaccines were mRNA, viral vector, and inactivated virus with two-dose schedules. With waning immunity after complete vaccination of COVID-19 (Fonseca, de Souza et al., 2022) and some evidence of reduced effectiveness against new variants (Andrews et al., 2022), many countries offered booster doses. In Brazil, the CoronaVac (Sinovac) was the first vaccine approved for emergency use and the third dose was administered, preferably, with the BNT162b2 vaccine (Pfizer/BioNTech). Previously, we showed that the third dose with the BNT162b2 vaccine boosted the anti-spike antibody titers in recipients of CoronaVac (Fonseca, Pinto et al., 2022). Now, we evaluated in the current cohort, the antibody levels after six months of the booster dose and whether the omicron wave in Brazil impacted the antibody response.