Booster dose of BNT162b2 after two doses of CoronaVac improves neutralization of SARS-CoV-2 Omicron variant

Brazil experienced high COVID-19 burden with extensive transmission and mortality but also implemented a large-scale vaccination program with strong public adherence. CoronaVac (inactivated vaccine) was widely used with a two-dose schedule. With the emergence of VOCs, particularly Delta and Omicron, the Brazilian program adopted an mRNA BNT162b2 booster to enhance protection. This study investigates whether a BNT162b2 booster after a two-dose CoronaVac primary series improves neutralizing antibody responses against the Omicron variant in a cohort of health care workers, addressing concerns about immune escape by Omicron and the effectiveness of heterologous boosting strategies.

The discussion references emerging evidence on heterologous vaccination strategies. Khang et al. reported higher immunogenicity when BNT162b2 followed CoronaVac, including against Omicron. Cheung et al. observed approximately 80% seroconversion (24/30) with the same combination, similar to the 76.6% found here. GeurtsvanKessel et al. showed that neutralizing antibodies against Omicron were lower or absent after several primary schemes but increased after a BNT162b2 booster across different vaccine platforms; T-cell responses were maintained across schemes, indicating Omicron’s larger impact on antibody neutralization than cellular immunity.

Design and participants: A total of 90 health care workers from two hospitals in Belo Horizonte (Minas Gerais, Brazil), who received a two-dose CoronaVac primary series in early 2021, were randomly selected and divided into three groups (n=30 each): D30 (sera collected 30 days after second CoronaVac dose), D60 (60 days after second dose), and D270 (270 days after second dose and 30 days after a BNT162b2 booster). Cohort demographics: 22 male, 68 female; ages 18 (n=21), 31–50 (n=56), 51–62 (n=13). Comorbidities present in 24 and absent in 66; prior infection in 11 vs no prior infection in 79. Vaccines: CoronaVac two-dose protocol (two 0.5 mL shots; 600 μF dose) followed by a BNT162b2 booster (0.3 mL; 30 μg spike mRNA). Ethics: Approved by Ethical Review Committee (CAAE 2898621.90000.5091); written informed consent obtained. Viral microneutralization assay (VNT50): Vero CCL-81 cells (10^4 per well) were seeded in 96-well plates and incubated 24 h at 37 °C, 5% CO2 to 90–95% confluence. Heat-inactivated sera (56 °C for 30 min) were serially diluted in DMEM with penicillin (100 U/mL), streptomycin (100 μg/mL), and 2% FBS; eight dilutions tested (1:20, 1:40, 1:80, 1:60, 1:320, 1:640, 1:280, 1:480). Diluted sera were incubated with live SARS-CoV-2 Omicron variant (HIAE-W.A.) at 50 TCID50/mL for 1 h at 37 °C, then added to cell plates and incubated 72 h at 37 °C, 5% CO2. After incubation, wells were fixed with 10% formaldehyde and stained with crystal violet. Neutralization capacity was defined as the reciprocal serum dilution preventing 50% cytopathic effect (VNT50), calculated using the Spearman-Karber algorithm. Each sample and dilution was tested in triplicate. Controls: A known Omicron-neutralizing serum served as positive control; a pre-pandemic serum as negative control. Statistics: For each time point, 30 biologically independent samples were analyzed. One-way ANOVA with Tukey’s multiple comparisons test was used; p < 0.05 considered significant.

• Among CoronaVac recipients, detectable neutralizing antibodies against Omicron were observed in 16.6% at 30 days (D30) and 10% at 60 days (D60) after the second dose, indicating waning and low neutralization against Omicron. - After a BNT162b2 booster (D270; 30 days post-booster), seroconversion rose to 76.6%. - The mean neutralization titer (VNT50) against Omicron increased 43.1-fold after the booster compared with the CoronaVac-only protocol. - Total N = 90; three equal groups of 30 evaluated at D30, D60, and D270.

The findings demonstrate that a heterologous BNT162b2 booster following a two-dose CoronaVac primary series substantially enhances neutralizing antibody responses against the SARS-CoV-2 Omicron variant. This addresses concerns about Omicron’s immune evasion by showing that boosting with an mRNA vaccine encoding the ancestral spike can elicit antibodies capable of recognizing conserved spike regions despite Omicron’s mutations. The results align with other reports indicating improved serological responses and higher seroconversion rates with this heterologous schedule, and with broader evidence that BNT162b2 boosters increase neutralization against Omicron across various primary vaccine platforms. While Omicron more profoundly impacts antibody neutralization than cellular immunity, the booster restores neutralizing activity to a level likely to improve protection at the population level, complementing prior observations of enhanced clinical effectiveness with this strategy.

A BNT162b2 mRNA booster after two doses of CoronaVac markedly increases seroconversion and neutralizing antibody titers against the Omicron variant, supporting heterologous boosting as an effective strategy to counter Omicron-associated immune escape. These data contribute to evidence informing vaccine policy in settings where CoronaVac was widely used. Future research should assess durability of boosted responses, breadth across emerging variants, correlates with clinical effectiveness, and cellular immunity in this cohort.