Neutralization assays that can measure neutralizing antibodies in serum are vital for large-scale serodiagnosis and vaccine evaluation. Here, we establish multiplexed lab-on-a-chip bioassays for testing antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and its variants. Compared with enzyme-linked immunosorbent assay (ELISA), our method exhibits a low consumption of sample and reagents (10 μL), a low limit of detection (LOD: 0.08 ng/mL), a quick sample-to-answer time (about 70 min), and multiplexed ability (5 targets in each of 7 samples in one assay). We can also increase the throughput as needed. The concentrations of antibodies against RBD, D614G, N501Y, E484K, and L452R/E484Q-mutants after two doses of vaccines are 6.6 ± 3.6, 8.7 ± 4.6, 3.4 ± 2.8, 3.8 ± 2.8, and 2.8 ± 2.3 ng/mL, respectively. This suggests that neutralizing activities against N501Y, E484K, and L452R/E484Q-mutants were less effective than RBD and D614G-mutant. We performed a plaque reduction neutralization test (PRNT) for all volunteers. Compared with PRNT, our assay is fast, accurate, inexpensive, and multiplexed with multiple-sample processing ability, which is good for large-scale serodiagnosis and vaccine evaluation.