ABSTRACT
OBJECTIVES: To determine the impact of a booster dose on the humoral response in individuals inoculated with a complete schedule of any SARS-CoV-2 vaccine, we evaluated the neutralizing antibody (NAb) titers of homologous or heterologous booster doses over a 90-days period in CoronaVac vaccinees from three centers in Santiago, Chile. METHODS: Individuals previously inoculated with two doses of CoronaVac (N = 523) were recruited in the context of the REFUERZO clinical trial (NCT04992182) and received either placebo (N=129), or a booster dose of CoronaVac (N=134), BNT162b2 (N=133) or ChAdOx1 (N=127). Pseudovirus neutralizing antibody titers (pVNT) were determined at baseline (day 0) as well as days 14, 30, 60 and 90 post booster administration. RESULTS: Inoculating a booster dose increases the pVNTs titers at days 14 and 30 in all groups, (13.5 and 12.0 fold increase for the CoronaVac group; 247.0 and 212.3 fold increase for the BTN162b2 group; and 89.1 and 128.1 fold increased for ChAdOx1 at each time point, respectively) with a decline observed at days 60 and 90. However, while pVNTs remained significantly higher for the BTN162b2 and ChAdOx1 groups at days 60 and 90, neutralizing antibody titers reached baseline levels in the CoronaVac group at 90 days post-inoculation. CONCLUSIONS: A single heterologous booster (BTN162b2 or ChAdOx1) in individuals who completed the CoronaVac primary series resulted in an important increase in NAb titers remaining significantly higher at least for 90 days. These data may directly impact middle- and low-income countries currently using CoronaVac as the main vaccination strategy.